[lit] Pass the USERPROFILE variable through on Windows

When running in a Windows Container, the Git for Windows Unix tools
(C:\Program Files\Git\usr\bin) just hang if this variable isn't
passed through.

Currently, running the LLVM/clang tests in a Windows Container fails
if that directory is added to the path, but succeeds after this change.
(After this change, the previously used GnuWin tools can be left out
entirely, too, as lit automatically picks up the Git for Windows tools
if necessary.)

Differential Revision: https://reviews.llvm.org/D98858