support passing custom ssh options via COLMENA_SSHOPTS

[yu-re-ka]

Like NIX_SSHOPTS but for all the ssh calls done by colmena.

Reasoning: Sometimes it is necessary to pass flags that are not part of the declarative deployment configuration. Examples are local user -l or ProxyJump.

[yu-re-ka]

Seeing very weird behavior where the "pushing system closure" part works, but the "activating system configuration" fails with permission denied, even though the specified username part is correct in the error message.