Check if shader cache directory is available before using cache

[clayjohn]

Might help: #79115

At any rate, this is necessary to allow users to turn off the shader cache.

We use shader_cache_dir_valid as a shorthand for if the shader cache is enabled and if the cache location is readable/writable in RD. we should be using the same structure here

CC @ChibiDenDen

[RandomShaper]

Makes sense.

[YuriSizov]

Thanks!