.github: Add differential-shellcheck workflow for shell script analysis

Introduce a new GitHub workflow to run shellcheck on changed shell
scripts. This workflow automatically detect and highlight potential
shell script issues in pull requests. This change is a follow-up to
commit 0700b32 which fixed an undefined variable issue in `install.sh`.
It intends to leverage static analysis to improve script quality and
catch potential errors early.

Shellcheck will now:
- Analyze all shell scripts modified in pull requests
- Provide inline comments with specific issue details
- Help prevent similar variable-related mistakes in the future

See also
https://github.com/redhat-plumbers-in-action/differential-shellcheck

Signed-off-by: Kefu Chai <[email protected]>

.github/workflows/differential-shellcheck.yaml

@@ -0,0 +1,32 @@
+---
+# https://github.com/redhat-plumbers-in-action/differential-shellcheck#readme
+
+name: Differential ShellCheck
+on:
+  push:
+    branches:
+      - master
+  pull_request:
+    branches:
+      - master
+
+permissions:
+  contents: read
+
+jobs:
+  lint:
+    runs-on: ubuntu-latest
+
+    permissions:
+      security-events: write
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Differential ShellCheck
+        uses: redhat-plumbers-in-action/differential-shellcheck@v5
+        with:
+          severity: warning
+          token: ${{ secrets.GITHUB_TOKEN }}