rpmbuild: fix make_srpm with new git
The new GIT doesn't allow us to run 'git archive' (typically done in
make_srpm method) as a privileged root user from a directory owned by
non-privileged user. Till we have #534 fixed, we need to do this hack
for make_srpm method (method is run under the root user in a mock
chroot).
Fixes: #2194
Merges: #2201