flake,go: use GOPROXY=direct

* both for the flake-provided shell environment and the flake-provided
  go module build env

inspired by:
  https://drewdevault.com/2022/05/25/Google-has-been-DDoSing-sourcehut.html

flake.nix

@@ -66,6 +66,10 @@
             ];
             nativeBuildInputs = [ pkgconfig ];
 
+            overrideModAttrs = _: {
+              GOPROXY = "direct";
+            };
+
             inherit version;
             doCheck = false;
             # use go.mod for managing go deps, instead of vendor-only dir
@@ -79,7 +83,7 @@
 
             modSha256 = lib.fakeSha256;
             # dont't forget to update vendorSha256 whenever go.mod or go.sum change
-            vendorSha256 = "sha256-LvdcTbj8cFlaIBsq1VLfLF2Tu9HiZzGO8igD766nMLE=";
+            vendorSha256 = "sha256-gaIoacCs0PcvmFW8dQOFUb9KU1u6KUiFmGQ/ERcVwjc=";
 
             # In 'nix develop', we don't need a copy of the source tree
             # in the Nix store.
@@ -151,6 +155,7 @@
               GOLDFLAGS = "-s -w -X cmd.version=${version}";
               CGO_CFLAGS = "-g2 -Og -mtune=generic";
               CGO_LDFLAGS = "-Wl,-O1,-sort-common,-as-needed,-z,relro,-z,now,-flto -pthread";
+              GOPROXY = "direct";
 
               shellHook = ''
                 echo " -- in go-xkcdreader shell..."