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/reusable-test.yml

@@ -85,8 +85,9 @@ jobs:
           key: ccache-${{ inputs.runs-on }}-${{ inputs.cc }}-${{ inputs.suite }}
 
       - name: Install test dependencies
-        run: |
-          ./ci/ci-install.bash test
+        uses: ./repo/.github/actions/install-deps
+        with:
+          stage: test
 
       - name: Set up Python venv
         uses: ./repo/.github/actions/setup-venv