util/glsl2spirv: only copy the input file if we're going to change it

Should make the build slightly faster by only copying when we actually
need to, and otherwise pointing at the original source

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/30132>

src/util/glsl2spirv.py

@@ -146,6 +146,9 @@ def postprocess_file(args: 'Arguments') -> None:
 
 
 def preprocess_file(args: 'Arguments', origin_file: T.TextIO, directory: os.PathLike, filemap: T.Dict[str, str]) -> str:
+    if args.create_entry is None and args.glsl_ver is None:
+        return origin_file.name
+
     with open(os.path.join(directory, os.path.basename(origin_file.name)), "w") as copy_file:
         lines = origin_file.readlines()