Bump version to 6.4.1

• Diff: https://gitlab.com/gitlab-org/gitlab-qa/compare/v6.4.0...7ab0243312d5ef7c143e381f29188db358408826

• Release notes:

I think it's open to debate whether this is a fix or a minor change given that it adds functionality. I chose "fix" because the intention was to use CI_JOB_URL instead of CI_JOB_ID, and the only user of the code is also changing, and it actually fixes some warnings.

### Fixes

- !543 Avoid method redefinition and pass CI_JOB_URL

• Checklist before merging:

  • Diff link is up-to-date.
  • Based on the diff, lib/gitlab/qa/version.rb is updated, according to SemVer.
  • Release notes are accurate.

• Checklist after merging:

  • Create a tag for the new release version.