Do not use an explicit user for ssh when deploymentUser is null

[dminuoso]

Right now colmena will statically emit the current user in the user@host target, preventing the usage of .ssh/config or equivalent of overriding the username.

In situations where a repository is shared with multiple people such that configuring deploymentUser in the repository is unfeasible, this makes it easier when certain remote machines demand a particular user that is different from the local user.

This also addresses #61

[zhaofengli]

Thanks! Could you also update the cargoSha256 in package.nix so nix-build works (see error for instructions)? To make things easier, we can also move back to cargoLock.lockFile since we are no longer pulling the git version of indicatif.

[dminuoso]

Done!

[zhaofengli]

Thanks again!