Re-run post-install-commands when synchronizing dependencies

[spiiph]

When running a command in an environment, or when entering a shell in the environment, dependencies are synchronized. Commands specified in [post-install-commands] don't seem to be run, though. I consider these commands to be part of the dependencies and expect them to run. Alternatively I would like to run the post-install commands manually. I would be willing to contribute both to this feature and to #650.