contribcheck and modulescheck of MSVC's vcregress.pl cannot work independently

Hi all,

I have just noticed that the commands contribcheck and modulescheck
will fail as follows if the temporary installation is not done in
$ROOT_DIR/tmp_install first:
C:\Users\ioltas\git\postgres\src\tools\msvc>vcregress contribcheck
============================================================
Checking btree_gin
The system cannot find the path specified.

All the other commands call InstallTemp() so as to have them work
independently. For consistency I think that we should do the same for
them. Attached is a patch to fix that.
Regards,
--
Michael