Actually the on-disk cache can store not only GLSL compiled shaders, but also SPIR-V, and even full Vulkan pipelines.
So the MESA_GLSL_CACHE a bit misleading, and MESA_SHADER_CACHEis probably a better name, specially when then default cache directory is ~/.cache/mesa_shader_cache.
This MR changes this envvar and all the references to it.