radv: make sure gs copy shader is retrieved from the cache with the variant
radv: make sure gs copy shader is retrieved from the cache with the variant

Apps can limit the size of the cache via VkAllocationCallbacks so we
can't be sure that both are always in the cache.

Reviewed-by: Bas Nieuwenhuizen <1d31d94f30d40df7951505d1034e1e923d02ec49@basnieuwenhuizen.nl>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler