Fix bootstrap for windows

[mibac138]

Since #71819 x.py doesn't work for me. This PR fixes that. To be honest I'm kind of confused how this didn't get caught by CI. Error:

Traceback (most recent call last):
  File "x.py", line 11, in <module>
    bootstrap.main()
  File "C:\Users\mibac\IdeaProjects\rust\src\bootstrap\bootstrap.py", line 960, in main
    bootstrap(help_triggered)
  File "C:\Users\mibac\IdeaProjects\rust\src\bootstrap\bootstrap.py", line 925, in bootstrap
    build.build = args.build or build.build_triple()
  File "C:\Users\mibac\IdeaProjects\rust\src\bootstrap\bootstrap.py", line 731, in build_triple
    return default_build_triple()
  File "C:\Users\mibac\IdeaProjects\rust\src\bootstrap\bootstrap.py", line 184, in default_build_triple
    ostype = require(["uname", "-s"], exit=required).decode(default_encoding)
AttributeError: 'NoneType' object has no attribute 'decode'

r? @Mark-Simulacrum who reviewed the previous PR

[retep998]

CI has MinGW installed and in PATH, even for pc-windows-msvc builds. It would be nice to have a builder that has no msys/mingw stuff in PATH whatsoever.

Also isn't this technically a dupe of #71964 ?

[mibac138]

Oh, I didn't notice that pr. Thanks