targets/va: override LIBVA_DRIVERS_PATH in devenv

This is really the right thing to do; it makes libva pick up the driver
we just built in a meson devenv. Not doing so can be kinda fatal;
otherwise we can end up loading different gallium-drivers for OpenGL and
VA-API, which can end up being pretty fatal.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27514>

src/gallium/targets/va/meson.build

@@ -46,6 +46,8 @@ if with_glx == 'xlib'
   link_with_libva_gallium += [libws_xlib]
 endif
 
+devenv.set('LIBVA_DRIVERS_PATH', meson.current_build_dir())
+
 if host_machine.system() == 'windows'
   link_with_libva_gallium += [libwsgdi]