Now that go/flutter-build works for flutter/plugins and flutter/packages has support for flutter/plugins and flutter/packages, I noticed that our Windows post-submit tests for stable are all failing, and apparently have been for a long time (forever?). We didn't notice because of LUCI tests not reporting post-submit results to GitHub (I'm not sure why, but that's always been true).

Example:
https://ci.chromium.org/ui/p/flutter/builders/prod/Windows%20win32-platform_tests%20stable/1669/overview

Every failure is:

fatal: reference is not a tree: <some hash>

I'm not sure why this only affecting stable checkout. Perhaps a recipe error, or a problem with the mirror?