Fix bug #8098

Fix bug #8098

The Windows shell cannot have a UNC path as the current directory.
However, using pushd to change to the same directory automatically
maps the UNC path to a drive letter and thus it works.

Note that when calling cmd.exe from a UNC path there are still
warnings about the UNC path, but the command is then successfully
executed because the current directory is set to something digestible
by the shell.
(cherry picked from commit b907a0dc7cca2e6037e0493589fe4b8f77d5a18e)