ci: call shellcheck on the bash script

Sadly, shellcheck does not support zsh [1], and it's not even possible
to evaluate the script with zsh because it fails with:
  _arguments:comparguments:327: can only be called from completion function
So the zsh script shall not be checked.

[1] koalaman/shellcheck#809

.github/workflows/ci.yml

@@ -78,10 +78,11 @@ jobs:
         ./builddir/mkosi -h
         ./builddir/mkosi documentation
 
-    - name: Test shell scripts
+    - name: Run shellcheck on scripts
       run: |
         sudo apt-get update && sudo apt-get install --no-install-recommends shellcheck
         bash -c 'shopt -s globstar; shellcheck bin/mkosi tools/*.sh'
+        bin/mkosi completion bash | shellcheck -
 
     - name: Test man page generation
       run: pandoc -s mkosi.md -o mkosi.1