Change Shellcheck GitHub action to cancel already running actions on same branch.

.github/workflows/shellcheck.yml

@@ -2,6 +2,12 @@ on:
   - push
   - pull_request
 
+# Cancels all previous workflow runs for the same branch that have not yet completed.
+concurrency:
+  # The concurrency group contains the workflow name and the branch name.
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
 jobs:
   shellcheck:
     runs-on: ubuntu-latest