ci: migrate to built in shellcheck action (#77)

Signed-off-by: Olly Pomeroy <opomer@amazon.co.uk>

Author: ollypom

.github/workflows/ci.yml

@@ -8,6 +8,6 @@ jobs:
   shellcheck:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
       - name: Run ShellCheck
-        uses: ludeeus/action-shellcheck@master
+        run: find . -type f -name "*.sh" -exec shellcheck {} +