fix(security): use self-repository syntax for setup-backend in testcontainers workflow

The `Setup Python` step in the testcontainers workflow references
`.github/actions/setup-backend/` via the workspace-relative `uses: ./...`
syntax, which zizmor's `self-repository` audit flags in favor of GitHub's
dedicated `uses: $/...` syntax. Unlike the workspace-relative form, the
self-repository form isn't subject to runtime filesystem state and is
treated by GitHub as a pinned reference.

`setup-backend` is a plain in-repo directory (not a git submodule), so the
mechanical rewrite applies cleanly here, matching the already-merged fixes
for the same pattern elsewhere in the workflow suite (#43975, #44018,
#44040). The ASF allowlist check was bumped to allowlist-check/v1.0.1 in
#44014 (merged), which recognizes the `$/` prefix, so this rewrite doesn't
break that required check.

Resolves code-scanning alert #2685

Co-Authored-By: Evan Rusackas <evan@preset.io>
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.github/workflows/testcontainers.yml

@@ -119,7 +119,7 @@ jobs:
         with:
           persist-credentials: false
       - name: Setup Python
-        uses: ./.github/actions/setup-backend/
+        uses: $/.github/actions/setup-backend/
         with:
           python-version: current
       - name: Install db2 driver (ibm-db-sa)