rpmbuild: disable bootstrap chroot for make_srpm method
There are two misconceptions that might cause that make_srpm can not
work with bootstrap chroot easily:
- the option --plugin-option=bind_mount:dirs=... is used
- nspawn_args is overwritten, with dropped privileges (so installing of
normal chroot isn't possible from bootstrap chroot)
This is not first time we are bitten by the fact that we don't use the
same mechanism for all source mehtods.