fix(live): Run shellcheck in CI to find errors in the scripts

[lslezak]

Problem

• There might be typos (like in Fixed syntax error in 99agama-cmdline dracut module #2029) or mistakes in the Live scripts.

Solution

• Run shellcheck in the CI and fail if any error is reported
• Because shellcheck reports lots of warning and some of them are false positives let's run it in two stages:
  • The first run checks only for errors, if an error is found then the CI fails
  • If no error is found it runs it in full mode with warnings enabled. But in this case the CI always passes.
• Later we should fix the warnings and silence the false positives and make the check to fail also with warnings.

Testing

• Tested manually and in GitHub Actions
• The first run found an error and failed
• After fixing the errors (there is actually one more), then it reports several warnings but the CI succeeds

[lslezak]

See https://www.shellcheck.net/wiki/SC2144 for more details about the problem.

[teclator]

LGTM