GH workflow: introduce shellcheck job

This job uses shellcheck to analyze all our bash scripts except for
those in /vendor directory.

It also allows sourcing /etc/os-release without checking it, because
it doesn't make any sense to check that the CI machine has correct
/etc/os-release file. Also allow useless cats because I don't think they
cause any harm.

.github/workflows/tests.yml

@@ -91,3 +91,14 @@ jobs:
 
       - name: Send coverage to codecov.io
         run: bash <(curl -s https://codecov.io/bash)
+  shellcheck:
+    name: "🐚 Shellcheck"
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v2
+    - name: Run ShellCheck
+      uses: ludeeus/action-shellcheck@0.5.0
+      with:
+        ignore: vendor # We don't want to fix the code in vendored dependencies
+      env:
+        SHELLCHECK_OPTS: -e SC1091 -e SC2002 # don't check /etc/os-release sourcing and allow useless cats to live inside our codebase