mkNugetDeps: unpack to source/

Unpacking to the build root was a bad idea. stdenv uses dumpVars() to
create a file env-vars containing the entire environment.  This was
being installed in the derivation output, and since it contains lots of
store paths, it was bloating the closure for every nuget package.

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

@@ -39,8 +39,9 @@ lib.makeOverridable(
           ];
 
           unpackPhase = ''
-            unzip -nq $src
-            chmod -R +rw .
+            unzip -nqd source $src
+            chmod -R +rw source
+            cd source
           '';
 
           prePatch = ''