Use env to run the Python-based tools

Otherwise overriding the Python interpreter with `--with-python` won't
work correctly.

gobject/glib-genmarshal.in

@@ -1,4 +1,4 @@
-#!@PYTHON@
+#!/usr/bin/env @PYTHON@
 
 # pylint: disable=too-many-lines, missing-docstring, invalid-name
 

gobject/glib-mkenums.in

@@ -1,4 +1,4 @@
-#!@PYTHON@
+#!/usr/bin/env @PYTHON@
 
 # If the code below looks horrible and unpythonic, do not panic.
 #