dcrdex: fix build id

pkgs/bisonw.nix

@@ -1,18 +1,21 @@
 {
   lib,
   buildGoModule,
-  fetchFromGitHub,
+  fetchgit,
+  git,
 }:
 
 buildGoModule (finalAttrs: {
   pname = "bisonw";
   version = "1.0.6";
 
-  src = fetchFromGitHub {
-    owner = "decred";
-    repo = "dcrdex";
+  # Keep the checkout metadata: Bison Wallet reads vcs.revision from Go build
+  # info for its Build ID and frontend cache-busting version.
+  src = fetchgit {
+    url = "https://github.com/decred/dcrdex.git";
     rev = "v${finalAttrs.version}";
-    hash = "sha256-MPHi+mqmumxZ/pFxBYxz73M6imi0GONV0M3raPq0yoI=";
+    hash = "sha256-IlJzvL0ZaeQ+BaKGArS6J5Wk2wctOVba7Xg8qjVI8KY=";
+    leaveDotGit = true;
   };
 
   subPackages = [
@@ -26,6 +29,15 @@ buildGoModule (finalAttrs: {
 
   vendorHash = "sha256-KCof0GFFaNWgMb2yR4l4lyeVZdiJSIyn4mgZ/InMZhk=";
 
+  nativeBuildInputs = [ git ];
+  GOFLAGS = [ "-buildvcs=true" ];
+
+  # fetchgit does not retain the Git index. Restore it in the writable build
+  # tree so Go does not mark the otherwise pristine checkout as modified.
+  preBuild = ''
+    git reset --mixed HEAD
+  '';
+
   meta = with lib; {
     description = "Bison Wallet - Multi-coin wallet with built-in DEX trading";
     homepage = "https://github.com/decred/dcrdex";
@@ -33,4 +45,3 @@ buildGoModule (finalAttrs: {
     mainProgram = "bisonw";
   };
 })
-