add shell linter shellcheck

This adds a GitHub action which runs a shell linter on all the .sh files.
ludeeus/action-shellcheck is just an action which relies on
https://github.com/koalaman/shellcheck which is a well known linter.
Fix only serious errors by now, we'll take care of warnings later.

    ~/src/libbpf-ci$ find -type f -name '*.sh' |xargs shellcheck -S error
    ~/src/libbpf-ci$ echo $?
    0

Signed-off-by: Matteo Croce <[email protected]>

.github/workflows/lint.yaml

@@ -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: -S error