feat: Add shellcheck as a step of GitHub action

* Added two new steps of "lint-code" job
  * Try to install shellcheck CLI tool during CI
  * Try to check rhc.bash using this tool during CI

.github/workflows/lint.yml

@@ -36,6 +36,10 @@ jobs:
         with:
           version: "latest"
           args: "--verbose"
+      - name: Install shellcheck
+        run: sudo apt install shellcheck
+      - name: Check shell bash scripts
+        run: shellcheck ./rhc.bash
   lint-language:
     name: Lint language
     runs-on: ubuntu-latest