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/coverage.yml

@@ -116,8 +116,9 @@ jobs:
           ls -lsha
 
       - name: Install dependencies
-        working-directory: repo
-        run: ./ci/ci-install.bash coverage
+        uses: ./repo/.github/actions/install-deps
+        with:
+          stage: coverage
 
       - name: Download code coverage data
         uses: actions/download-artifact@3e5f45b2cfb9172054b4087a40e8e0b5a5461e7c  # v8