Image builder fails with "useradd: group '###' does not exist"

Issue

Image builder fails to create a blueprint that specifies both a user and a group with the same id.

useradd: group '999' does not exist
Traceback (most recent call last):
  File "/run/osbuild/bin/org.osbuild.users", line 171, in <module>
    r = main(args["tree"], args["options"])
  File "/run/osbuild/bin/org.osbuild.users", line 160, in main
    useradd(tree, name, uid, gid, groups, description, home, shell, password)
  File "/run/osbuild/bin/org.osbuild.users", line 103, in useradd
    subprocess.run(["chroot", root, "useradd", *arguments, name], check=True)
  File "/usr/lib64/python3.9/subprocess.py", line 528, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['chroot', '/run/osbuild/tree', 'useradd', '--uid', '999', '-o', '--gid', '999', '--comment', 'Default', '--home-dir', 'home/user', '--create-home', '--shell', '/usr/bin/bash', 'user']' returned non-zero exit status 6.