CI: Add a bash linter to pre-submits

Warns about shellcheck issues with severity `error`.

.github/workflows/lint.yml

@@ -0,0 +1,20 @@
+name: "Lint github actions"
+
+on:
+  push:
+    branches:
+      - master
+
+name: "Trigger: Push action"
+permissions: {}
+
+jobs:
+  shellcheck:
+    name: Shellcheck
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master
+        with:
+          severity: error