Reprotest has only two variations enabled

After commit fda81fe3 (cc @santiago), reprotest only has user_group and domain_host variations enabled. All other variations are disabled. This can be seen e.g. here:

https://salsa.debian.org/python-team/packages/turbosearch/-/jobs/4412522

INFO:reprotest:build "experiment-1": FIX environment, FIX build_path, FIX kernel, FIX aslr, FIX num_cpus, FIX time, vary user_group, FIX fileordering, vary domain_host, FIX home, FIX locales, FIX exec_path, FIX timezone, FIX umask

It is because that commit used --variations option which overwrites the variations list. I think it should have used --vary option which is the same, but appends to previous values instead of overwriting them.