Run QA GDK tests against commit SHA

What does this MR do and why?

When checking out the code to test, this uses the pipeline commit SHA instead of the ref slug. We want to test merged result pipelines and the commit ref slug only includes changes in the source branch.

Part of #390758 (closed)

Screenshots or screen recordings

Before:

• In an example MR (with branch jmd/handle-dismissal-reason-ff), the top of the test-on-gdk-smoke job shows the checkout as:

  Checking out 27c502ef as refs/merge-requests/117431/merge... 

• But when the GDK container runs it checks out a different ref:

  HEAD is now at 102742b2e4fb Merge branch 'winniehell-mergerequest-favicon' into 'master'
  fatal: couldn't find remote ref jmd-handle-dismissal-reason-ff
  error: pathspec 'jmd-handle-dismissal-reason-ff' did not match any file(s) known to git

After:

• In this MR the test-on-gdk-smoke job shows the checkout as:

  Checking out ca73dea8 as refs/merge-requests/117489/merge...

• And the GDK container checks out the same ref:

  HEAD is now at ca73dea834e9 Merge branch 'ml-qa-gdk-checkout-commit-sha' into 'master'

(side note: we shouldn't need to check out the code again in the container since it's already available in the job execution context. That's going to be fixed as part of #390758 (closed))

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.