Description of problem:
The build fails (any fedora-rawhide-x68_64 build, but it happened for rhelbeta-8 BR too).

mock, locally, gives me the same error. Using `--old-chroot` and ommiting `--bootstrap-chroot` fixed that(not sure which one of those).

I probably have boostrap-chroot `on` for my repos- I'll try disabling.

How reproducible:
fedora-rawhide-x86_64 buildroot, but it could be random

Steps to Reproduce:
1. submit a build to fedora-rawhide-x86_64

Actual results:
https://copr-be.cloud.fedoraproject.org/results/pvalena/vagrant/fedora-rawhide-x86_64/01016139-vagrant/
https://copr-be.cloud.fedoraproject.org/results/pvalena/vagrant/rhelbeta-8-x86_64/01016139-vagrant/
https://copr-be.cloud.fedoraproject.org/results/pvalena/rubygems/fedora-rawhide-x86_64/01016393-rubygem-puma/

```
Finish(bootstrap): dnf install
ERROR: Exception(/tmp/tmpkin4y4va/vagrant.spec) Config(1016139-fedora-rawhide-x86_64) 0 minutes 57 seconds
INFO: Results and/or logs in: /var/lib/copr-rpmbuild/results
INFO: Cleaning up build root ('cleanup_on_failure=True')
Start: clean chroot
INFO: unmounting tmpfs.
Finish: clean chroot
ERROR: Could not find useradd in chroot, maybe the install failed?
WARNING: Could not find required logging config file: /var/lib/copr-rpmbuild/results/configs/logging.ini. Using default...
INFO: mock.py version 1.4.16 starting (python version = 3.7.4)...
Start(bootstrap): init plugins
INFO: tmpfs initialized
INFO: selinux disabled
INFO: chroot_scan: initialized
INFO: compress_logs: initialized
Finish(bootstrap): init plugins
Start: init plugins
INFO: tmpfs initialized
INFO: selinux disabled
INFO: chroot_scan: initialized
INFO: compress_logs: initialized
Finish: init plugins
Start: run
Start: scrub ['cache']
INFO: scrubbing cache for child
Finish: scrub ['cache']
Finish: run

Traceback (most recent call last):
  File "/usr/bin/copr-rpmbuild", line 129, in main
    action(args, config)
  File "/usr/bin/copr-rpmbuild", line 254, in build_rpm
    builder.run()
  File "/usr/lib/python3.7/site-packages/copr_rpmbuild/builders/mock.py", line 39, in run
    self.produce_srpm(spec, self.sourcedir, configdir, self.resultdir)
  File "/usr/lib/python3.7/site-packages/copr_rpmbuild/builders/mock.py", line 98, in produce_srpm
    raise RuntimeError("Build failed")
RuntimeError: Build failed
```