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

@@ -1,4 +1,4 @@
-name: WPT export
+name: Pull request (WPT export)
 on:
   pull_request_target:
     types: ['opened', 'synchronize', 'reopened', 'edited', 'closed']

.github/workflows/pull-request.yml

@@ -0,0 +1,23 @@
+name: Pull request
+on:
+  pull_request:
+    types: ['opened', 'synchronize']
+    branches: ["**"]
+
+
+jobs:
+  build-linux:
+    name: Linux
+    if: github.repository == 'servo/servo'
+    uses: ./.github/workflows/linux.yml
+    with:
+      layout: '2013'
+      unit-tests: true
+
+  build-linux-layout-2020:
+    name: Linux (layout-2020)
+    if: github.repository == 'servo/servo'
+    uses: ./.github/workflows/linux.yml
+    with:
+      layout: '2020'
+      unit-tests: false

.github/workflows/quick-check.yml

@@ -1,6 +1,8 @@
+# This action is meant as a quick check to be run when pushing to
+# branches on forks. This allows forks to test the build and unit
+# tests automatically without opening a pull request.
 name: Quick check
 on:
-  pull_request:
   push:
     branches:
       ["**", "!master", "!auto", "!try", "!try-linux", "!try-mac", "!try-windows", "!try-wpt", "!dependabot/**"]
@@ -8,6 +10,7 @@ on:
 jobs:
   build-linux:
     name: Linux
+    if: github.repository != 'servo/servo' || github.event_name == 'workflow_dispatch'
     uses: ./.github/workflows/linux.yml
     with:
       layout: '2013'
@@ -15,6 +18,7 @@ jobs:
 
   build-linux-layout-2020:
     name: Linux (layout-2020)
+    if: github.repository != 'servo/servo' || github.event_name == 'workflow_dispatch'
     uses: ./.github/workflows/linux.yml
     with:
       layout: '2020'