Show `EMPIRE_*` env vars in `emp env`

[ejholmes]

This was brought up internally. It could be nice to pull in the EMPIRE_* env vars into emp env. Some are process specific (e.g. EMPIRE_PROCESS, EMPIRE_PROCESS_SCALE) so we can't include those, but seeing EMPIRE_APPID, EMPIRE_APPNAME, EMPIRE_RELEASE could help with visibility.

Also, if someone tries to emp set those, we should error out. Right now, they could be set, but they'll still get overridden before being passed to the scheduling backend.