Fix an accidental Module copy in build.zig.

build.zig

@@ -224,7 +224,7 @@ pub fn build(b: *std.Build) anyerror!void {
  else => true,
  };
 
- var exe_mod = exe_step.root_module;
+ const exe_mod = &exe_step.root_module;
 
  exe_mod.addImport("graf", graf_mod);
  exe_mod.addImport("clap", clap_mod);