fix(ci): pin Node via setup-node in bump-gitstream-core workflow (#532)

The Bump gitstream-core workflow had no actions/setup-node step, so it ran
on the runner's preinstalled Node 20. Newer @linearb/gitstream-core requires
node>=22 and its native dep isolated-vm fails to compile against Node 20's
V8 headers. Add setup-node@v6 with node-version-file: .nvmrc to match the
other workflows in this repo.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: MishaKav

.github/workflows/bump-gitstream-core.yml

@@ -36,6 +36,10 @@ jobs:
         with:
           token: ${{ secrets.CI_USER_GIT_TOKEN }}
 
+      - uses: actions/setup-node@v6
+        with:
+          node-version-file: .nvmrc
+
       - name: Set version to ENV
         run: |
           echo "VERSION=${{ inputs.version }}" >> "$GITHUB_ENV"