We could get rid of MANPATH, if all man pages install to $out/share/man.
Assume a suitable PATH is set, try:
  $ guix package -r wireless-tools
  $ env MANPATH= manpath
Get:
  /run/current-system/profile/man:/home/iyzsong/.guix-profile/share/man
It's because wireless-tools having man pages in $out/man, after apply
this patch, we should get:
  /run/current-system/profile/share/man:/home/iyzsong/.guix-profile/share/man

Then `man' will just work even without MANPATH.