tarpaulin: install via taiki-e/install-action, fix libssl.so.1.1 load failure

[guaardvark]

Problem

The tarpaulin workflow's check (coverage) job has been red on every master push since 2025-10-19 — independent of the source, which compiles and tests cleanly (cargo test --all-features is green, all tests pass).

actions-rs/tarpaulin@v0.1 downloads a pinned cargo-tarpaulin 0.22.0 binary that is linked against libssl.so.1.1. The ubuntu-24.04 runner only ships libssl.so.3, so the binary cannot load and exits before any test runs:

[tarpaulin] downloading cargo-tarpaulin from .../0.22.0/cargo-tarpaulin-0.22.0-travis.tar.gz
cargo-tarpaulin: error while loading shared libraries: libssl.so.1.1:
    cannot open shared object file: No such file or directory
##[error]The process '/home/runner/.cargo/bin/cargo' failed with exit code 127

Coverage therefore never executes. actions-rs/* is also archived/unmaintained and already emits a Node 20 deprecation warning on every run.

Fix

Install a current cargo-tarpaulin through the maintained taiki-e/install-action and invoke it directly. --out Xml and the existing arguments are preserved, so the downstream codecov/codecov-action upload is unchanged.

-      - uses: actions-rs/tarpaulin@v0.1
+      - uses: taiki-e/install-action@v2
         with:
-          version: '0.22.0'
-          args: '--all-features --exclude-files src/lib.rs -- --test-threads 1'
+          tool: cargo-tarpaulin
+      - run: >-
+          cargo tarpaulin --out Xml --all-features
+          --exclude-files src/lib.rs -- --test-threads 1

This is a CI-only change; no library code is touched.