create stamp file for clippy in Config::download_clippy

Due to missing stamp file, we were downloading (and applying nix patches if enabled)
continuously every time `Config::download_clippy` was called. This change fixes that by
creating stamp file at the end of the function.

Signed-off-by: onur-ozkan <work@onurozkan.dev>

src/bootstrap/src/core/download.rs

@@ -399,6 +399,7 @@ impl Config {
             self.fix_bin_or_dylib(&cargo_clippy.with_file_name(exe("clippy-driver", host)));
         }
 
+        self.create(&clippy_stamp, date);
         cargo_clippy
     }