fix(ci): serialize and prune target caches

tools/ci-matrix/src/ci_gate.rs

@@ -1067,9 +1067,14 @@ fn validate_workflows(root: &Path, manifest: &CoverageManifest) -> Result<()> {
         "push:",
         "pull_request:",
         "timeout-minutes: 30",
-        "cancel-in-progress: true",
+        "group: ci-${{ github.workflow }}",
+        "cancel-in-progress: false",
         "runs-on: ubuntu-latest",
         "required-gate",
+        "flock --exclusive 9",
+        "rm -f -- \"$cargo_target/.metacrate-ready\"",
+        "rm -f -- \"$CARGO_TARGET_DIR/.metacrate-ready\"",
+        "stale_target",
         "python3 tools/normalize_git_mtimes.py --state",
         ".metacrate-mtimes.json\" --check",
         "'**/Cargo.toml'",