fs: make symlinking more resilient when something's already at the location

modules/fs/default.nix

@@ -298,7 +298,10 @@ let
       lnfrom="$1"
       lnto="$2"
 
-      ln -sf --no-dereference "$lnto" "$lnfrom"
+      # ln is clever when there's something else at the place we want to create the link
+      # only create the link if nothing's there or what is there is another link,
+      # otherwise you'll get links at unexpected fs locations
+      ! test -e "$lnfrom" || test -L "$lnfrom"  && ln -sf --no-dereference "$lnto" "$lnfrom"
     '';
     scriptArgs = [ path link-cfg.target ];
   };