scons: fix SPIR-V -> NIR build

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
Tested-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Fixes: 18f9fc91 ('spirv: add and use a generator id enum')
Part-of: <!7096>
82 jobs for !7096 with scons_vtn_generator_ids in 14 minutes and 56 seconds (queued for 9 seconds)
merge request