[octalide]

ci.yml builds its compiler from one pinned mach commit instead of mach dev head, and caches it. This is the same change as briar-systems/mach-std#1032.

• env gains MACH_REF: 5470b803eecb044d3c907419a97a54dcee51993b.
• The compiler step fetches exactly that commit, never a branch.
• actions/cache keys the built compiler on mach-${{ runner.os }}-${{ env.MACH_VERSION }}-${{ env.MACH_REF }}. A hit skips the seed install and the build. Adding the compiler to PATH is its own step, so it runs on both paths.
• A later bump is a chore: build with mach <short sha> pull request.

Decisions

• The workflow is mach-std#1032's step verbatim, except the comment says "the package" where std's says "std".
• actions/cache@v6, only $RUNNER_TEMP/mach cached, and the seed release in the key, all as in mach-std#1032.

Ran

actionlint on ci.yml. CI on this head built the compiler on a cache miss and passed.