make-nuget-deps: support an url field in fetchNuGet

If a package source defines an url field, use it instead of the
url based on package name and version, which assumes nuget.org as
a package source.

pkgs/build-support/dotnet/make-nuget-deps/default.nix

@@ -1,9 +1,10 @@
 { linkFarmFromDrvs, fetchurl }:
 { name, nugetDeps }:
-  linkFarmFromDrvs "${name}-nuget-deps" (nugetDeps {
-    fetchNuGet = { pname, version, sha256 }: fetchurl {
+linkFarmFromDrvs "${name}-nuget-deps" (nugetDeps {
+  fetchNuGet = { pname, version, sha256
+    , url ? "https://www.nuget.org/api/v2/package/${pname}/${version}" }:
+    fetchurl {
       name = "${pname}-${version}.nupkg";
-      url = "https://www.nuget.org/api/v2/package/${pname}/${version}";
-      inherit sha256;
+      inherit url sha256;
     };
-  })
+})