Motivation for this change

Fixes GitLab repository mirroring using ssh (new feature, was only https before). When trying to sync, I was receiving the following error:

Fetching remote remote_mirror_c187a6534238bb8e69db3948dfe1d8cd failed: ssh '-oIdentityFile="/tmp/gitlab-shell-key-file20190130-1398-1w4swpe"' '-oIdentitiesOnly="yes"' '-oStrictHostKeyChecking="yes"' '-oUserKnownHostsFile="/tmp/gitlab-shell-known-hosts20190130-1398-fdoq0r"':  command not found
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
and the repository exists.

command not found was referring to ssh. The GitLab repository mirroring is handled by Gitaly. Adding openssh to Gitaly fixed the problem.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.