Revert "Upgrade shellcheck before using it"

This reverts commit 5e83eee793.

.github/workflows/shellcheck.yml

@@ -16,11 +16,6 @@ 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