Cache the cargo-llvm-cov binary in the coverage job

Skips the compile on cache hits; version is pinned so the cache key
matches what gets installed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.forgejo/workflows/ci.yml

@@ -102,12 +102,19 @@ jobs:
             sccache-${{ runner.os }}-
 
       # Not baked into the rust-ci image (see docker/rust-ci in
-      # infrastructure) — install fresh each run rather than bumping the
-      # shared image just for this one project.
+      # infrastructure) — cache the compiled binary rather than bumping
+      # the shared image just for this one project. To upgrade, change the
+      # version here, in the cache key, and in the install command below.
+      - name: Cache cargo-llvm-cov binary
+        uses: actions/cache@v4
+        with:
+          path: ~/.cargo/bin/cargo-llvm-cov
+          key: cargo-llvm-cov-${{ runner.os }}-0.9.1
+
       - name: Install cargo-llvm-cov
         run: |
           rustup component add llvm-tools-preview
-          command -v cargo-llvm-cov >/dev/null 2>&1 || cargo install cargo-llvm-cov --locked
+          command -v cargo-llvm-cov >/dev/null 2>&1 || cargo install cargo-llvm-cov --locked --version 0.9.1
 
       # Reports coverage only — no --fail-under-lines yet. main.rs is
       # excluded: thin argv dispatch exercised by the real end-to-end