go: merge package src links with mergeAttrsList

The GOPATH src symlinks were combined with a linear foldl' of //.
mergeAttrsList is the equivalent shallow merge and scales with the package
count without quadratic recopies.

modules/programs/go.nix

@@ -161,7 +161,7 @@ in
 
               mkSrc = n: v: { "${mainGoPath}/src/${n}".source = v; };
             in
-            lib.foldl' (a: b: a // b) { } (lib.mapAttrsToList mkSrc cfg.packages)
+            lib.mergeAttrsList (lib.mapAttrsToList mkSrc cfg.packages)
           );
         }