Use forward slashes on Windows for git exclude - #50662

Description

git clean -xffd --exclude=bin\cache\ wasn't working on Windows:

C:\Windows\Temp\flutter sdk>call git clean -xffd --exclude=bin\cache\ 
Removing bin/cache/

git wants forward slashes for this flag, even on Windows.

Related Issues

Pulled out from #50527

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Did any tests fail when you ran them? Please read [Handling breaking changes].

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.