This commit unites two scripts (.sh + .bat) in one .ps1. PS was choosed, since:
1) We don't need MSYS2 to distribute with Inno at first. Now, who wants to
just distribute GIMP can do it easier natively without a separate environ.
2) PS is actually pretty solid and have a good number of cmdlets, unlike the
ancient CMD, which barely supported more than 9 parameters in .bat files.
Consequently, some redundant variable checks have been removed from the .ISS.
Also, changes the name of the job and script for better consistency with the
upcoming MS Store job.
e61b7e05ed