Pin checkout version. (#1121)

We are updating a single version to test the theory that when we are
mixing versions and hashes the update is calculated once and reused
across all the files causing the hashes to be replaced with versions.

.github/workflows/beta.yml

@@ -22,7 +22,7 @@ jobs:
       matrix:
         os: [ubuntu-latest, macos-latest, windows-latest]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@5f87f43226edfc1a08fc4c0d26875fa13e3540c4
       - uses: actions/setup-java@v1
         with:
           java-version: '12.x'