nixos/borgbackup: install job wrappers to systemPackages for easy borg access

Currently to run borg job manually, you have to use systemctl:
```
$ systemctl start borgbackup-job-jobname.service
```

This commit makes wrappers around borg jobs available in $PATH, which have
BORG_REPO and connection args set correctly:
```
$ borg-job-jobname list
$ borg-job-jobname mount ::jobname-archive-2019-12-25T00:01:29 /mnt/some-path
$ borg-job-jobname create ::test /some/path
```

Closes: https://github.com/NixOS/nixpkgs/pull/64888


Co-authored-by: Danylo Hlynskyi <abcz2.uprola@gmail.com>