ponysay: make sure we use python3

It requires python3, but uses `/usr/bin/env python` in the she-bang,
which is ambiguous.  Setting the environment to `python3` fixes the
current run-time error.

srcpkgs/ponysay/template

@@ -1,7 +1,7 @@
 # Template file for 'ponysay'
 pkgname=ponysay
 version=3.0.3
-revision=1
+revision=2
 noarch=yes
 hostmakedepends="python3"
 depends="python3"
@@ -16,5 +16,6 @@ do_install() {
 	python3 setup.py --prefix=/usr --dest-dir=$DESTDIR \
 		--freedom=partial --everything \
 		--with-pdf=/usr/share/doc/ponysay --without-pdf-compression \
+		--with-custom-env-python=python3 \
 		install
 }