fix: upgrade actions/cache from v4 to v5 (Node 24)

Node.js 20 actions are deprecated — actions/cache v4 uses Node 20.
Upgrade to v5 which supports Node 24.

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

Author: pftg

action.yml

@@ -81,7 +81,7 @@ runs:
 
     - id: load-cache
       if: ${{ env.CACHE_KEY }}
-      uses: actions/cache/restore@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
+      uses: actions/cache/restore@v5
       with:
         path: ~/cache-apt-pkgs
         key: cache-apt-pkgs_${{ env.CACHE_KEY }}
@@ -117,7 +117,7 @@ runs:
 
     - id: save-cache
       if: ${{ env.CACHE_KEY && ! steps.load-cache.outputs.cache-hit }}
-      uses: actions/cache/save@0057852bfaa89a56745cba8c7296529d2fc39830 # v4.3.0
+      uses: actions/cache/save@v5
       with:
         path: ~/cache-apt-pkgs
         key: ${{ steps.load-cache.outputs.cache-primary-key }}