CI: Cache archives downloaded by apt (#8159)

Belt-and-braces in case GitHub apt mirrors start acting up again: On any
successful apt install, cache the deb files downloaded/needed for the
step, so on next run the mirrors need not be consulted. (That is: trade
apt mirror access with GitHub cache access, which is more reliable)

.github/workflows/format.yml

@@ -27,7 +27,9 @@ jobs:
           token: ${{ secrets.GITHUB_TOKEN }}
 
       - name: Install packages for build
-        run: ./ci/ci-install.bash format
+        uses: ./repo/.github/actions/install-deps
+        with:
+          stage: format
 
       - name: Configure git identity
         run: |