move "source /etc/profile" from .zlogin to .zprofile

[Meiyo Peng]

Hello,

Guix's default skeletons put "source /etc/profile" into ~/.zlogin. But
/etc/profile resets the PATH environment variable, exports many other
environment variables, and source /etc/bashrc. According to
http://zsh.sourceforge.net/Intro/intro_3.html .zlogin is sourced after
.zshrc and should not be used for alias definitions, options,
environment variable settings, etc. Especially "as a general rule, it
should not change the shell environment at all". I think it's better to
source /etc/profile from ~/.zprofile.

Meiyo Peng