Add $MKOSI_USER, $MKOSI_GROUP and $MKOSI_HOME

[4nd3r]

In specific situations, I need to access the login name, primary group name, and the user's home directory ($HOME) within the postinst script. Right now, I have to pass these values through the command line and list them in mkosi.conf. However, having these values readily accessible by default would make things simpler for me and could be beneficial for others as well.

If someone more familiar with the code base thinks it's good idea, I'm willing to try and submit PR myself.