Revert "rootbld: run mkusers inside the chroot and don't affect host users"

We can't run mkusers inside the chroot because of bwrap peculiarities.
Presently, rootbld does therefore not work at all for APKBUILDs which
use $pkggroups/$pkguser. While not polluting the host is a noble goal
it isn't really useful if it causes rootbld to be incapable of building
certain packages (i.e. those using $pkggroups/$pkguser with groups
not existent on the host).

This commit therefore restores the original behavior for now. While at
it, I also added a comment to the mkusers invocation.

See: #10094

This reverts commit 84d7b769.