gst1-python3: make sure python3 is used

According to @jnbr this change should fix the package which
erroneously seems to detect python2 when being configured.

Closes: #10432

srcpkgs/gst1-python3/template

@@ -1,7 +1,7 @@
 # Template file for 'gst1-python3'
 pkgname=gst1-python3
 version=1.12.4
-revision=2
+revision=3
 wrksrc="gst-python-${version}"
 build_style=gnu-configure
 configure_args="--disable-static"
@@ -15,3 +15,7 @@ license="LGPL-2.1"
 homepage="https://gstreamer.freedesktop.org"
 distfiles="${homepage}/src/gst-python/gst-python-${version}.tar.xz"
 checksum=20ce6af6615c9a440c1928c31259a78226516d06bf1a65f888c6d109826fa3ea
+
+pre_configure() {
+	export PYTHON=/usr/bin/python3
+}