Reorganize the workflow files for pull requests a little.

Split out the quick check GitHub action (which is really only meant for
forks). This prevents the quick check from running twice for PRs from
branches on the upstream repository. Also rename the WPT export action
file to make it clearer that it runs for pull requests.

.github/workflows/pull-request-wpt-export.yml

@@ -0,0 +1,40 @@
+name: Pull request (WPT export)
+on:
+  pull_request_target:
+    types: ['opened', 'synchronize', 'reopened', 'edited', 'closed']
+
+jobs:
+  upstream:
+    # Run job only on servo/servo
+    if: github.repository == 'servo/servo'
+    runs-on: ubuntu-latest
+    steps:
+      - name: Calculate PR fetch depth
+        run: echo "PR_FETCH_DEPTH=$(( ${{ github.event.pull_request.commits }} + 1 ))" >> "${GITHUB_ENV}"
+      - name: Check out shallow servo PR
+        run: |
+          mkdir servo
+          cd servo
+          git init -b main
+          git remote add origin ${{ github.event.repository.clone_url}}
+          git fetch origin pull/${{ github.event.pull_request.number}}/head:pr --depth ${{ env.PR_FETCH_DEPTH }}
+          git fetch origin master:master --depth 1
+          git checkout master
+      - name: Check out wpt
+        uses: actions/checkout@v3
+        with:
+          path: wpt
+          repository: 'web-platform-tests/wpt'
+          # The token here must be the token that we will use to push to the
+          # WPT repository and not the token used for GitHub actions, because
+          # the checkout action sets up an `extraheader` authorization override
+          # using the token specified here.
+          # See https://github.com/actions/checkout/issues/162.
+          token: ${{ secrets.WPT_SYNC_TOKEN }}
+      - name: Install requirements
+        run: pip install -r servo/etc/ci/upstream-wpt-changes/requirements.txt
+      - name: Process pull request
+        run: servo/etc/ci/upstream-wpt-changes/upstream-wpt-changes.py
+        env:
+          GITHUB_CONTEXT: ${{ toJson(github) }}
+          WPT_SYNC_TOKEN: ${{ secrets.WPT_SYNC_TOKEN }}