meson: Replace external_program.path() with external_program.full_path()

meson: Replace external_program.path() with external_program.full_path()

The path() method is deprecated in 0.55.0 and we're recommended
to use full_path() instead. Interestingly, we were already doing
do in couple of places, but not all of them.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>