configure: expose PYTHON to test/tcg/config-host.mak

This will be useful for making $shell calls to something more flexible
than the shell builtins.

Reviewed-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-ID: <20250725154517.3523095-11-alex.bennee@linaro.org>

configure

@@ -1800,6 +1800,7 @@ echo "SRC_PATH=$source_path" >> tests/tcg/$config_host_mak
 if test "$plugins" = "yes" ; then
     echo "CONFIG_PLUGIN=y" >> tests/tcg/$config_host_mak
 fi
+echo "PYTHON=$python" >> tests/tcg/$config_host_mak
 
 tcg_tests_targets=
 for target in $target_list; do