gettext: remove cross suffix from envHook

`role_post` is meant for cross compilation, eg. `CC_FOR_BUILD` and
`CC_FOR_TARGET`. There's no such concept for `GETTEXTDATADIRS`.

In particular we hit this with `cantarell-fonts`, where only
`GETTEXTDATADIRS_FOR_BUILD` is defined due to empty buildInputs.

see NixOS#50855

pkgs/development/libraries/gettext/gettext-setup-hook.sh

@@ -2,7 +2,7 @@ gettextDataDirsHook() {
     # See pkgs/build-support/setup-hooks/role.bash
     getHostRoleEnvHook
     if [ -d "$1/share/gettext" ]; then
-        addToSearchPath "GETTEXTDATADIRS${role_post}" "$1/share/gettext"
+        addToSearchPath GETTEXTDATADIRS "$1/share/gettext"
     fi
 }