[ci] Check out master when managing tags & labels (#18393)

* [ci] Check out master when managing tags & labels

Previously, we identified a problem when attempting to run a GitHub
Action for the "pull_request" event when the underlying branch had been
deleted [1]. By migrating to the latest GitHub Action beta release [2],
we've since gained more insight into the process and more control over
its behavior.

Alter the configuration for the new "checkout" Step to consistently
check out the `master` branch. Update the Action specifier to reference
a stable release of the Action.

[1] #18286
[2] #18359

* fixup! [ci] Check out master when managing tags & labels

.github/workflows/pull_request.yml

@@ -4,7 +4,10 @@ jobs:
   update-pr-preview:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@master
+    - uses: actions/checkout@v1.0.0
+      with:
+        ref: refs/heads/master
+        fetch-depth: 1
     - name: update-pr-preview
       uses: ./tools/docker/github
       env: