Pass the --update-head-ok to fetch for PRs out of main

[nkammah]

It seems like docs PR coming from a local main branch fail to checkout on Buildkite with the error:
fatal: Refusing to fetch into current branch refs/heads/main of non-bare repository

This PR attempts solves the issue by using the --update-head-ok flag - see explanation in this stack overflow.

Note - this also addresses the issue where status checks were not reported when the build was failing

This was tested using one of the problematic PR:

• reproducing the failure
• with the fix (additional flag) (but then aborting the build)

[github-actions]

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[bmorelli25]

Awesome. Thanks!