[gtk-osx] Set $PYTHON in the jhbuild environment to force using python3.

[John Ralls <jralls src gnome org>]

commit 232ba3f155188cd4c5170bf8a62da42476afb0eb
Author: John Ralls <jralls ceridwen us>
Date:   Sat Mar 7 17:49:44 2020 -0800

    Set $PYTHON in the jhbuild environment to force using python3.
    
    Set it to "/usr/bin/env python3" so that it will work regardless of
    whether it's the built or the virtenv python3.

 jhbuildrc-gtk-osx | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/jhbuildrc-gtk-osx b/jhbuildrc-gtk-osx
index fa2f3a0..5e0fb98 100644
--- a/jhbuildrc-gtk-osx
+++ b/jhbuildrc-gtk-osx
@@ -595,7 +595,7 @@ if _python_ver:
     append_autogenargs('libxml2',
                        '--with-python-install-dir=' + _python_install_path)
     environ_append('PYTHONPATH', _python_install_path, ':')
-
+os.environ['PYTHON'] = '/usr/bin/env python3'
 # Unset this so we don't mess with the check for not starting
 # recursively.
 os.unsetenv("JHB")