Add options to the nix package manager installer for better integration with other distros

[mpedramfar]

There are two improvements that come to my mind that I think would be good to have as options within the install script, or if not, at least deserve some explanation in the documentation.

1. Desktop files for GUI applications:

This has been mentioned before #3910 and the answer was that basically in a default installation, we don't want to change too many things out of fear of breaking the host system. What I'm suggesting is either an option in the install script or better documentation so that users won't have search through github issues or stack exchange to add this functionality.

2. sudo vs. sudo -i:

As I understand, sudo clears environment variables for security reasons. In particular, it sets the PATH variable to a default value specified in the /etc/sudoers file. Because of that, simply running sudo command doesn't work with anything installed with nix, since it can't find the executables. One solution is to simply modify sudoers file which can be done within the install script as a non-default option. The alternative to changing sudoers file, as I understand, is to replace all instances of sudo command with sudo -i command in scripts and other places which is not ideal.