work around git locking issue on Windows

[geekosaur]

Simply disabling or removing the step causes all non-Windows to fail, so we now skip it only on Windows.
This replaces Artem's PR #11086, which GitHub refused to let me reopen for some reason.

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Template B: This PR does not modify behaviour or interface

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[geekosaur]

Looks successful, @ulysses4ever.

[ulysses4ever]

Thanks!

[Mikolaj]

Well done!

[geekosaur]

I should probably mention that I set priority: high because it's a CI fix (we hope!)… but the fact that several PRs went in with only a Mac bobble before this landed suggests it might not be necessary and possibly only looks like it solved the problem. ☹️

[geekosaur]

I've just discovered something odd: this isn't present at all in validate-actions, yet its CI passes (all the tests involving local Git checkouts to e.g. test source-repository-package should be failing, per #8546). Additionally, disabling that for Windows didn't cause any new tests to fail . At the time I wondered if we were already skipping all such tests on Windows for some reason, but now I'm suspicious. Especially given my previous comment.