wireless-regdb: fix build when python2 from package feeds exists

wireless-regdb: fix build when python2 from package feeds exists

wireless-regdb fails to build if there is python2 installed from package
feeds, as staging_dir/hostpkg/bin/python is python2 and
staging_dir/hostpkg/bin takes precedence over staging_dir/host/bin
(proper place with python -> python3 symlink) which leads to the build
failure of wireless-regdb, so this patch makes it explicit which python
should be used.

Reported-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>
Tested-by: Russell Senior <russell@personaltelco.net>
Tested-by: Lucian Cristian <lucian.cristian@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit b6bae4a2c9f11f7e55319c2b4c709396ce649688)