fix(actions): authenticate snapcraft before nightly remote build (#38252)

The `release-nightly-snapcraft` workflow’s `build-and-publish` job was
failing because `snapcraft remote-build` fell back to interactive
Launchpad authorization in CI. This change makes authentication explicit
and non-interactive before the remote build step.

- **Workflow change**
  - Add an `Authenticate snapcraft` step before `Remote build`.
- Run `snapcraft login --with` using the existing
`SNAPCRAFT_STORE_CREDENTIALS` secret.
  - Pin that step to `shell: bash` to support process substitution.

- **Why this fixes the failure**
  - Prevents CI from entering browser-based Launchpad auth flow.
  - Ensures `remote-build` runs with preloaded credentials.

```yaml
- name: Authenticate snapcraft
  shell: bash
  env:
    SNAPCRAFT_STORE_CREDENTIALS: ${{ secrets.SNAPCRAFT_STORE_CREDENTIALS }}
  run: snapcraft login --with <(printf '%s' "$SNAPCRAFT_STORE_CREDENTIALS")
```

---------

Signed-off-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

.github/workflows/release-nightly-snapcraft.yml

@@ -22,6 +22,10 @@ jobs:
       - name: Install snapcraft
         run: sudo snap install snapcraft --classic
 
+      - name: Authenticate snapcraft
+        shell: bash
+        run: snapcraft login --with <(printf '%s' "$SNAPCRAFT_STORE_CREDENTIALS")
+
       - name: Remote build
         run: |
           snapcraft remote-build \