Use SystemRoot instead of WINDIR, to fix tox

In most cases they will be the same, but WINDIR may be absent (or
have a different value?) in rare cases.

The practical reason it matters in GitPython's tests is that tox
automatically passes SystemRoot through to environments on Windows.

Author: EliahKagan

test/test_git.py

@@ -150,7 +150,7 @@ def test_it_executes_git_not_from_cwd(self, rw_dir, case):
         if os.name == "nt":
             # Copy an actual binary executable that is not git. (On Windows, running
             # "hostname" only displays the hostname, it never tries to change it.)
-            other_exe_path = os.path.join(os.getenv("WINDIR"), "system32", "hostname.exe")
+            other_exe_path = os.path.join(os.environ["SystemRoot"], "system32", "hostname.exe")
             impostor_path = os.path.join(rw_dir, "git.exe")
             shutil.copy(other_exe_path, impostor_path)
         else: