Fix and simplify CI caching

[julianspeith]

The reported failure

The Ubuntu 22.04 job on fix/documentation died at the Cache pip Linux step:

The template is not valid. .github/workflows/ubuntu22.04.yml (Line: 34, Col: 16):
hashFiles('**/requirements.txt') failed. Fail to hash files under directory '/home/runner/work/hal/hal'

hashFiles returns an error when the pattern resolves to zero files, and the job stops there — before Install Dependencies, Configure, Build or Test. requirements.txt is present on that branch at the same three paths as on master, the workflow files are byte-identical between the branches, and the trees differ only by six new .rst files and two .DS_Store files, so the branch content does not explain it. What does stand out is that the pattern makes the runner walk the entire ~4500 file workspace, deps/abc included, to find a file whose path is known.

This PR hashes the file directly, hashFiles('requirements.txt'), in all six live occurrences. Same cache key semantics, no workspace traversal.

A real caching bug found along the way

The three Linux workflows key their ccache on ${{ steps.ccache_cache_timestamp.outputs.timestamp }}, but only macOS.yml defines a Prepare ccache timestamp step. On Linux the key therefore evaluated to a constant, which the run logs confirm:

Build and Test on Ubuntu 22.04    ccache cache files
Cache not found for input keys: Linux-ccache-, Linux-ccache-

The primary key and the restore key are the same string. actions/cache skips saving when the primary key hits exactly, so once any successful run writes Linux-ccache-, every later run restores that same snapshot and never saves an updated one — the compiler cache freezes at its first contents and stops helping as the tree moves on.

Fixed by adding the missing timestamp step to all three Linux workflows, so the key is unique per run and the Linux-ccache- restore-key prefix picks up the most recent entry.

Other cleanups

• macOS.yml generated its timestamp with ::set-output, which GitHub deprecated and warns about on every run. It now writes to $GITHUB_OUTPUT. Both platforms use the same snippet and key format.
• Removed the Cache CCache step from the Linux workflows: it cached ~/.ccache while CCACHE_DIR points at ${{runner.workspace}}/.ccache, so it archived an empty directory, and its restore keys referenced an undefined env.cache-name.
• Removed the Cache pip macOS step from the Linux workflows, where startsWith(runner.os, 'macOS') can never be true.
• Added a concurrency group so a new push to a branch cancels the superseded run. Runs on master are exempt so the documentation deployment always completes.

All four workflows parse and keep their step order; only dead steps were removed.

Workflow modernization (second commit)

• actions/checkout@v1 (macOS) and @v2 (the rest) replaced with @v4. Both were deprecated and forced a Node 24 shim on every run. Since @v4 accepts fetch-depth, the three hand-written git fetch steps in the Linux workflows are gone; fetch-depth: 0 supplies the full history and tags that cmake/hal_cmake_tools.cmake needs for git describe --tags. That also removes git fetch --prune --unshallow, which is what failed run 31498874580 on master with fatal: shallow file has changed since we read it. macOS previously used @v1, which cloned full history implicitly, so fetch-depth: 0 preserves its behavior.
• push restricted to master, pull_request added. Previously every push to every branch started four workflows of roughly 50 minutes each, including a full documentation build on branches that can never deploy. Changes now get exactly one run per push via their pull request, and the Deploy Doc step stays gated on github.ref == 'refs/heads/master', which no pull_request event satisfies.
• -DPYBIND11_PYTHON_VERSION=3.6 dropped from the three Linux configure steps. That Python is long end-of-life, is not installed on any supported runner, and the variable is ignored by the pybind11 version HAL requires. It appears nowhere else in the tree.
• Single-entry build matrix replaced with a plain runs-on, and the resulting always-true startsWith(runner.os, ...) step conditions removed (2 in each Ubuntu workflow, 4 in releaseDoc.yml, 4 in macOS.yml).

Check names change

Dropping the matrix shortens the reported check names:

before	after
Build and Test on Ubuntu 22.04 (ubuntu-22.04)	Build and Test on Ubuntu 22.04
Build and Test on Ubuntu 24.04 (ubuntu-24.04)	Build and Test on Ubuntu 24.04
Build and Test macOS (macOS-latest)	Build and Test macOS
Build and release documentation (ubuntu-22.04)	Build and release documentation

Worth knowing while reviewing this: master's branch protection currently lists exactly one required status check, Build (macOS-latest), which no workflow in the repository produces — the macOS job is named Build and Test macOS. That required check can never report, so it is already stale today and this rename does not make it worse. Someone with admin rights may want to point it at the real check names above.

🤖 Generated with Claude Code