load nix.sh from ~/.profile and update XDG_DATA_DIRS

[jnoortheen]

Is your feature request related to a problem? Please describe.
gui applications installed with nix do not appear in linux DE's launcher. The problem is answered here

Describe the solution you'd like
this is added by nix upon install.

if [ -e ~/.nix-profile/etc/profile.d/nix.sh ]
then 
	source ~/.nix-profile/etc/profile.d/nix.sh
fi # added by Nix installer

it is better to be put in ~/.profile so that gui apps will pickup the path variable changes. Also adding Nix's data folder to XDG_DATA_DIRS like mentioned in the answer will make things easier for newcomers.

export XDG_DATA_DIRS=$HOME/.nix-profile/share:"${XDG_DATA_DIRS:-/usr/local/share/:/usr/share/}"

[matthewbauer]

/etc/profile and $HOME/.profile are not always loaded by the shell. Bash only loads /etc/bashrc and ~/.bashrc on non-login shells (although some systems source profile from those for you).

XDG_DATA_DIRS isn't added for fear of breaking the host system: #2443 (comment). It's not too hard to just add it yourself though.