Fix environments variables #35

`env(1)` says its use is:

env [OPTION]... [-] [NAME=VALUE]... [COMMAND [ARG]...]

but sshkit was forming commands as:

( [NAME=VALUE]... env [OPTION]... [-] [COMMAND [ARG]...] )

which lead to the environment being ignored when mixed with `sudo su`,
that was used to change the user executing the command.