Re-add missing environment variables

[AnyTimeTraveler]

Fixes #443 and #445

The function env_init in line 622 clears the whole environment.
After adding the environment variables generated in lines 508 and 509 back in, everything works as it should.

I don't know enough about the rest of the code to know if the lines 508 and 509 can now be removed, so I didn't remove them yet.
I'd like some help from someone more experienced, if the variables are needed anywhere between the first setting (line 508) and the clearing (line 622).

You can see in #425 (link to the specific line)that the function env_xdg used to be called before env_init.

[AnyTimeTraveler]

I just booted my PC a second time and had the same problem again, but this time sway is saying that there is no seat available.
I am going to investigate further before this should be merged.

[AnyTimeTraveler]

Okay, there must be some kind of race-condition here, as I have rebooted my PC 5 more times and each time logging into sway worked flawlessly.
For now, this pull request definitely corrects the resetting of the environment after XDG_RUNTIME_DIR is set.