Make flutter and dart scripts invoke their batch file equivalents on Windows (#59789)

This makes the flutter and dart scripts invoke their batch file equivalents if running under MINGW (i.e. git-bash) on Windows.

This allows for proper locking, and makes sure that people aren't using two different (and non-mutally-aware) locking systems when running flutter on Windows.

I also fixed a couple of places where we look for MINGW32, which fails under MINGW64. It just looks for MINGW now.