ci: enable shellcheck linter

Run shellckeck linter in a github action,
as in https://github.com/libbpf/ci/pull/61

Signed-off-by: Matteo Croce <teknoraver@meta.com>

.github/workflows/lint.yml

@@ -0,0 +1,19 @@
+name: "lint"
+
+on:
+  pull_request:
+  push:
+    branches:
+      - master
+
+jobs:
+  shellcheck:
+    name: ShellCheck
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v3
+      - name: Run ShellCheck
+        uses: ludeeus/action-shellcheck@master
+        env:
+          SHELLCHECK_OPTS: --severity=error