Track dependencies at the env level

[emillon]

As discussed in #1147 (comment), we should track when (env) changes and trigger a rebuild in that case.

[rgrinberg]

Another thing that will be useful here is to add a way to get the environment at the action level. Naturally, this should respect the setenv action.

[rgrinberg]

@emillon this can be closed now?

[emillon]

Yes, that's mostly taken care of by #1186. I think setenv works correctly since the env is passed. The rest will be taken care of by #1147.