flake(go): use go_1_18 everywhere

flake.nix

@@ -45,7 +45,7 @@
         inherit system;
         overlays = [
           # no overlay imports atm
-          (import ./overlay.nix)
+          # (import ./overlay.nix)
         ];
       });
   in rec {
@@ -59,10 +59,10 @@
       inherit (pkgs) lib overlays; # -> lib = pkgs.lib;overlays = pkgs.overlays;
     in rec {
       math-optim = with pkgs;
-        buildGoModule rec {
+        buildGo118Module rec {
           pname = "${projname}";
           buildInputs = [
-            go
+            go_1_18
             # gcc
             # glibc
             # glibc.static
@@ -226,7 +226,8 @@
               [
                 # use pathed go, as it's (supposed to be) faster with cgo,
                 # which will inevitably encountered anyway.
-                goPkgs.go
+                # goPkgs.go
+                go_1_18
                 go-tools
                 gopls
                 gofumpt