.zshrc: Export the PS1 instead of just setting it

I'm not sure if this is important actually, but I suppose we want
subshells to get the PS1?

Signed-off-by: Alan Orth <alan.orth@gmail.com>

.zshrc

@@ -32,7 +32,7 @@ alias ls='ls -F --color=auto'
 alias less='less -R' # preserves colors in GNU coreutils' `less`
 
 # Environment
-PS1='[%n@%m: %~]$ '
+export PS1='[%n@%m: %~]$ '
 export EDITOR=/usr/bin/vim
 export PAGER=/usr/bin/less