vulkan/pipeline: Always init pipeline cache objects

vk_shader_init_cache_obj() is fast enough and the already-found case is rare enough that there's no good reason to avoid the init. This allows us to use vk_shader_unref instead of vk_shader_destroy which is probably a touch safer over-all. It also fixes the assert that the two shaders have matching keys.

Closes: #10752 (closed)