MinGW platforms: duplicate headers cause compilation failures

[maleadt]

On MinGW platforms, both GCCBootstrap and PlatformSupport provide headers like pthread_time.h, resulting in potential compilation errors based on the order of these artifacts getting mapped in the contained. It's both problematic that our artifacts provide duplicate files, and that there's a nondeterminism in how artifacts get mapped.

For more details, see #9680 (comment)