Update action-hosting-deploy workflow template

[jhuleatt]

Description

Update the default workflow template for the Deploy to Firebase Hosting action:

• Don't run on forks. Github secrets aren't available to forks, so the action will always fail on a PR from a fork.
• Remove the preview flag because channels aren't in preview any more

We check to see if a PR is from a fork with:

if: '${{ github.event.pull_request.head.repo.full_name == github.repository }}'

I found the check in this community post, and @samtstern tested it in google/devlibrary#18

Scenarios Tested

firebase init hosting:github

Here's the diff between the old output of firebase-hosting-pull-request.yml and the new one:

# This file was auto-generated by the Firebase CLI
# https://github.com/firebase/firebase-tools

name: Deploy to Firebase Hosting on PR
'on': pull_request
jobs:
  build_and_preview:
+   if: '${{ github.event.pull_request.head.repo.full_name == github.repository }}'
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: FirebaseExtended/action-hosting-deploy@v0
        with:
          repoToken: '${{ secrets.GITHUB_TOKEN }}'
          firebaseServiceAccount: '${{ secrets.FIREBASE_SERVICE_ACCOUNT_MY_PROJECT }}'
          projectId: my-project
-       env:
-         FIREBASE_CLI_PREVIEWS: hostingchannels