ci: use unique channel ID per PR for preview deployments (#87)

Use 'pr-{number}' as the channelId instead of 'live' so each pull
request gets its own isolated preview deployment URL. This prevents
concurrent PR deployments from overwriting each other on the sandbox
Firebase project. Preview channels are automatically expired by
Firebase Hosting when the PR is closed.

Author: Kunal1522

.github/workflows/firebase-hosting-pull-request.yml

@@ -22,5 +22,5 @@ jobs:
         with:
           repoToken: '${{ secrets.GITHUB_TOKEN }}'
           firebaseServiceAccount: '${{ secrets.FIREBASE_SERVICE_ACCOUNT_MLAB_SANDBOX }}'
-          channelId: live
+          channelId: 'pr-${{ github.event.number }}'
           projectId: mlab-sandbox