ci: Update cache action

This change fixes deprecation warnings for Node.js 16 actions in the GHA
CI.

See:
- https://github.com/marketplace/actions/cache
- https://github.com/actions/cache/releases/tag/v4.0.0

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

@@ -16,7 +16,7 @@ runs:
  cat valgrind_fingerprint
  shell: bash
 
- - uses: actions/cache@v3
+ - uses: actions/cache@v4
  id: cache
  with:
  path: ${{ env.CI_HOMEBREW_CELLAR_VALGRIND }}