BLD: fix invalid shebang for build helper script

Shebangs do not work this way, but Meson could usually paper over the
differences by noticing that the last element is "python3" and rewiring
it to use its own sys.executable. However, this failed on WSL, likely
because platform quirks meant the script was directly executable, while
simultaneously having a shebang that was a fatal error.

Use a standard shebang indicating this script would like to run with
some form of python3 (which is exactly what it needs).

Fixes #17020

scipy/_build_utils/cythoner.py

@@ -1,4 +1,4 @@
-#!python3
+#!/usr/bin/env python3
 """ Scipy variant of Cython command
 
 Cython, as applied to single pyx file.