Fix after_script processing for Runners APIv4

What does this MR do?

Fixes a bug added with /api/v4/jobs/request endpoint.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

after_script step is encoded improperly in the payload. It should be an array of strings while it is an array of arrays of strings. This ends with errors on Runners side discussed in gitlab-ci-multi-runner#2272 and gitlab-ci-multi-runner#2271.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related to gitlab-ci-multi-runner#2272, gitlab-ci-multi-runner#2271