avoid re-uploading the docs when a PR is based on the upstream repo

In this case, we have the secret available, and the workflow ran even
though it wasn't on branch "master" because of the pull request trigger.

Since the change hasn't landed on master, though, we do not want to
update the website. So check for pushes to master, specifically.

.github/workflows/website.yml

@@ -41,7 +41,7 @@ jobs:
         run: |
           ssh-agent -a $SSH_AUTH_SOCK > /dev/null
           ssh-add - <<< "${{ secrets.WEBSITE_PRIV_KEY }}"
-        if: env.HAS_SSH_KEY == 'true'
+        if: github.ref == 'refs/heads/master' && env.HAS_SSH_KEY == 'true'
       - name: Build website
         run: |
           git config --global user.name "github-actions"
@@ -56,7 +56,7 @@ jobs:
         run: |
           cd docs
           ninja -C _build upload
-        if: env.HAS_SSH_KEY == 'true'
+        if: github.ref == 'refs/heads/master' && env.HAS_SSH_KEY == 'true'
       - name: Release the current JSON docs
         uses: svenstaro/upload-release-action@v2
         with:
@@ -71,4 +71,3 @@ jobs:
           file: docs/_build/meson-reference.3
           tag: ${{ github.ref }}
         if: ${{ github.event_name == 'release' }}
-