[debuginfo-tests] Don't look for Python 3 if we already have it

LLDB already requires Python 3 on Windows, so I already configure it
that way. For some reason CMake fails to find the one that Visual Studio
automatically installs at this standard location:
  C:/Program Files (x86)/Microsoft Visual Studio/Shared/Python37_64/python.exe

CMake prefers the python on path, which happens to be python 2.7.

Reviewers: aprantl, jmorse

Differential Revision: https://reviews.llvm.org/D69684