shellcheck: Run shellcheck as a Github action.

Add shellcheck as an action to be executed on every PR.

.github/workflows/lint.yml

@@ -76,3 +76,11 @@ jobs:
         run: |
             pip install pylint==2.10.2
             pylint plugins --disable=import-error
+
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@1.1.0