Make our own git checkout action

[AlekSi]

That would checkout PR's HEAD SHA instead of merge commit that is not available for pull_request_target.
It also should checkout a named branch.

[Balou9]

@AlekSi What other tasks are required to tackle this task?

[AlekSi]

This issue is not not ready Issues that are not ready to be worked on; PRs that should skip CI to be worked on