buildGo{Module,Package}: set GOTOOLCHAIN to local

prevent go from downloading another toolchain

pkgs/build-support/go/module.nix

···
   args = removeAttrs args' [ "overrideModAttrs" "vendorSha256" "vendorHash" ];
 
   GO111MODULE = "on";
+  GOTOOLCHAIN = "local";
 
   goModules = if (vendorHash == null) then "" else
   (stdenv.mkDerivation {
···
 
     inherit (args) src;
     inherit (go) GOOS GOARCH;
-    inherit GO111MODULE;
+    inherit GO111MODULE GOTOOLCHAIN;
 
     # The following inheritence behavior is not trivial to expect, and some may
     # argue it's not ideal. Changing it may break vendor hashes in Nixpkgs and
···
     inherit (go) GOOS GOARCH;
 
     GOFLAGS = lib.optionals (!proxyVendor) [ "-mod=vendor" ] ++ lib.optionals (!allowGoReference) [ "-trimpath" ];
-    inherit CGO_ENABLED enableParallelBuilding GO111MODULE;
+    inherit CGO_ENABLED enableParallelBuilding GO111MODULE GOTOOLCHAIN;
 
     configurePhase = args.configurePhase or (''
       runHook preConfigure

pkgs/build-support/go/package.nix

···
     inherit CGO_ENABLED enableParallelBuilding;
 
     GO111MODULE = "off";
+    GOTOOLCHAIN = "local";
     GOFLAGS = lib.optionals (!allowGoReference) [ "-trimpath" ];
 
     GOARM = toString (lib.intersectLists [(stdenv.hostPlatform.parsed.cpu.version or "")] ["5" "6" "7"]);