(gha) Update pr_testing to use latest checkout action version

Author: jpartlow

.github/workflows/pr_testing.yaml

@@ -13,7 +13,7 @@ jobs:
   shellcheck:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Install ShellCheck
         run: sudo apt-get install shellcheck
       - name: Run ShellCheck