fix: nix build environment fixes

- flake.nix: add capnpc source override (ghfast.top proxy)
- flake.nix: add GOROOT to devShell for Go version consistency
- tools/go_build: support file:// GOPROXY for sandbox builds
- bcctool: add .gitignore for gomodcache/
- bcctool: go.mod updated by go mod download

Co-Authored-By: Claude <noreply@anthropic.com>

src/flake.nix

@@ -22,6 +22,14 @@
           mypkgs = xbuild.packages.${system} // extralib.packages.${system} // librtti.packages.${system};
           overlays = [
             (final: prev: mypkgs // prev)
+            (final: prev: {
+              capnpc = prev.capnpc.overrideAttrs (oldAttrs: {
+                src = final.fetchurl {
+                  url = "https://ghfast.top/https://github.com/shen390s/c-capnproto/archive/e45ff682c4756ac5067cbac6d0f282a8ea5f0155.tar.gz";
+                  hash = "sha256-zsiX6unPF45eISP6sqbQImrivJuSE0BF4bTP/db+lQY=";
+                };
+              });
+            })
           ];
           pkgs = import nixpkgs { inherit overlays system;};
           xpkgs = import ./nixSupport/default.nix {
@@ -58,6 +66,7 @@
           defaultPackage = xpkgs.xcecore_debug;
           devShells.default = pkgs.mkShell {
             nativeBuildInputs = xpkgs.xcecore.nativeBuildInputs;
+            GOROOT = "${pkgs.go}/share/go";
           };
 
           devShell = self.devShells.${system}.default;