bug#68172: guix-module-union and guile modules take priority over $GUILE_LOAD_PATH

[Mikhail Tsykalov]

When running (bash syntax) "env GUILE_LOAD_PATH=/tmp:$GUILE_LOAD_PATH 
guix repl <(echo '(display %load-path) (newline)')" /tmp is placed after 
guix-module-union and guile modules.

While this may be desired behavior (judging from commit message of 
459f0d07a1b), it leads to the user being unable to overwrite modules in 
guix-module-union using environmental variables.

This doesn't really break anything documented, but it makes pre-inst-env 
with unbuilt scripts/guix pull modules from guix-modules-union instead 
of working directory. (i.e. "make check-system" will ignore changes to 
gnu/tests subdirectory). Also I think it's logical for user paths to 
take priority over system ones.

I think these entries are added by 
https://git.savannah.gnu.org/cgit/guix.git/tree/guix/self.scm?id=2a242e86379ebddbdddf2927f26e5e27a98fc605#n616.