python3-scons: fix regex replacing python by python3

Match only the first line and match even if the line says python3.
Otherwise we end up with multiple 3s at the end after multiple
invocations of do_install on same source.

(From OE-Core rev: 144c6c7183dd2b32c65edf325038eb84683a28f8)

Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/python/python3-scons_3.0.5.bb

@@ -15,7 +15,7 @@ UPSTREAM_CHECK_REGEX = "(?P<pver>\d+(\.\d+)+)\.tar"
 inherit setuptools3
 
 do_install_prepend() {
-    sed -i -e "s:/usr/bin/env python:/usr/bin/env python3:g" ${S}/script/*
+    sed -i -e "1s,#!.*python.*,#!${USRBINPATH}/env python3," ${S}/script/*
 }
 
 RDEPENDS_${PN} = "\