system: Check whether ~/.guix-profile/etc/profile exists.

* gnu/system.scm (etc-directory)[profile]: Check for
  ~/.guix-profile/etc/profile rather than just ~/.guix-profile.

gnu/system.scm

@@ -484,7 +484,7 @@ (define* (etc-directory #:key
 # Prepend setuid programs.
 export PATH=/run/setuid-programs:$PATH
 
-if [ -d \"$HOME/.guix-profile\" ]
+if [ -f \"$HOME/.guix-profile/etc/profile\" ]
 then
   # Load the user profile's settings.
   GUIX_PROFILE=\"$HOME/.guix-profile\" \\