Dependant default env variables

[viperML]

Right now, the Installable args use NH_FILE and NH_ATTRS for all subcommands. It should have an specific env variable for nh os and nh home.

[viperML]

My first idea is to apply the "default values and env variables" at the run site, instead at the clap parsing site.

The problem with this, is that clap would still show the same description for all commands.

[viperML]

I've implemented something in #179 but I'm not sure it's the best idea.

[viperML]

Another option would be to have an specific Installable variant for each subcommand, such that it includes the proper description OsInstallable, etc, and then map it into Installable according to all the fallback values