feat(gem-check): add --list flag to pass gem names for list check

[sanghanan]

• Related: https://github.com/chainguard-dev/internal-dev/issues/26219

gem-check derives the gem list -i target from the apk package name (stripping ruby[0-9.]+-). This assumes a 1:1 mapping between apk name and gem name, which is not true for packages that bundle multiple gems.

Example: ruby3.2-kube-logging-operator-fluentd-outputs is a collection of fluentd output plugin gems. The apk name is not itself a gem, so the list check fails:

  FAIL[gem-check]: Failed listing gem [kube-logging-operator-fluentd-outputs]
  PASS[gem-check]: Success requiring gem [fluent/plugin/out_oss]

To fix this, we add a --list flag to gem-check:

• --list foo / --list "foo bar" — check named gem(s) instead of the package-derived guess
• --list none — skip the list check entirely
• omitted/empty — current auto-detect behavior (so it won't break anything)

We could do something like:

- uses: test/tw/gem-check
  with:
    list: |
      fluent-plugin-oss
      fluent-plugin-foo
    require: |
      fluent/plugin/out_oss
      fluent/plugin/out_foo

[sanghanan]

(moving this to draft, skip none shouldnt be an option)