gha: Bump deprecated GHA workflow dependencies

Deprecation notice:
https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

Changelogs (no entries relevant to us):
  * https://github.com/docker/setup-buildx-action/releases/tag/v4.0.0
  * https://github.com/docker/build-push-action/releases/tag/v7.0.0
  * https://github.com/actions/cache#v5

.github/actions/install-homebrew-valgrind/action.yml

@@ -16,7 +16,7 @@ runs:
         cat valgrind_fingerprint
       shell: bash
 
-    - uses: actions/cache@v4
+    - uses: actions/cache@v5
       id: cache
       with:
         path: ${{ env.CI_HOMEBREW_CELLAR_VALGRIND }}