ci(coq): switch coq-build to noble apt — fix coqc-not-found unblocking ~5 PRs

.github/workflows/coq-build.yml

@@ -84,25 +84,30 @@ jobs:
     needs: detect-relevant-changes
     if: needs.detect-relevant-changes.outputs.relevant == 'true'
     runs-on: ubuntu-latest
-    # coq:8.18 matches the toolchain pinned by formal/Justfile and the
-    # local developer setup. Pin to a digest later once a stable
-    # known-good image is decided on; for now the floating tag mirrors
-    # `make` semantics — "what the maintainer's machine has".
-    container:
-      image: coqorg/coq:8.18
-      # actions/checkout's post-step writes to /__w/_temp/_runner_file_commands
-      # via Node fs.writeFileSync. The coqorg/coq image defaults to the
-      # non-root `coq` user (UID 1000), but the runner mounts /__w as the
-      # host runner user — EACCES until checkout's saveState. Running the
-      # container as root keeps the workspace-shared paths writable while
-      # leaving the in-container build untouched (root in container is
-      # still mapped to the runner host's runner user, not host-root).
-      options: --user root
+    # Coq 8.18 via Ubuntu 24.04 noble apt (coq 8.18.0+dfsg) exactly
+    # matches the toolchain pinned by formal/Justfile and the local
+    # developer setup.
+    #
+    # Why not the `coqorg/coq:8.18` container? Previously this job ran
+    # it with `--user root` to dodge the EACCES from checkout writing
+    # to a runner-user-owned /__w mount. But the image only configures
+    # `coqc` on the non-root `coq` user's PATH (via opam env in their
+    # .profile). With `--user root`, no opam env → `coqc: not found` →
+    # exit 127 immediately at the version step. Several proof PRs were
+    # silently blocked while main stayed green (path-filter skipped
+    # main pushes that didn't touch formal/). See the proven 37s
+    # pattern in [[reference_coqorg_image_apt_coq_simpler]] used by
+    # verisimdb#87.
 
     steps:
       - name: Checkout repository
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd  # v6.0.2
 
+      - name: Install Coq 8.18 (noble apt)
+        run: |
+          sudo apt-get update -qq
+          sudo apt-get install -y --no-install-recommends coq
+
       - name: Coq version (provenance)
         run: |
           coqc --version