back out the build_runner.zig moving change

I'd like to move this file but to do so requires a zig1.wasm update, so
I'll choose a more opportune moment to make this change.

src/main.zig

@@ -5388,7 +5388,6 @@ fn cmdBuild(gpa: Allocator, arena: Allocator, args: []const []const u8) !void {
             } else .{
                 .root = .{
                     .root_dir = zig_lib_directory,
-                    .sub_path = "compiler",
                 },
                 .root_src_path = "build_runner.zig",
             };