Cache the CI toolchain in a shared action

ci/toolchain installs the pinned tools, collections and rsync from cache for
both GitHub and Gitea; GitLab caches pip downloads. shellcheck now finds
scripts rather than globbing, which broke when a glob matched nothing.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/workflows/connectivity.yml

@@ -37,13 +37,8 @@ jobs:
     environment: production
     steps:
       - uses: actions/checkout@v4
-      - uses: actions/setup-python@v5
-        with:
-          python-version: "3.12"
       - name: Install the pinned toolchain
-        run: |
-          set -eu
-          scripts/check.sh --print-specs | head -1 | xargs -d '\n' pip install --quiet
+        uses: ./ci/toolchain
       - name: Load the deploy key
         env:
           SSH_KEY_BACKEND: ci