Upgrade shellcheck before using it

.github/workflows/shellcheck.yml

@@ -16,6 +16,11 @@ jobs:
       - name: Check out the codebase.
         uses: actions/checkout@v2
 
+      - name: Upgrade shellcheck to latest available release in ubuntu
+        run: |
+          sudo apt-get update
+          sudo apt-get install shellcheck
+
       - name: Shellcheck!
         run: |
           shellcheck migrate2rocky/migrate2rocky.sh