https://lists.freedesktop.org/archives/mesa-dev/2018-October/207343.html
Optimistically drop disk cache patch, changelog mentions related
changes to cache behavior.
Not sure if those changes address our problem
(can we count on build-id?) so someone more familiar with
this should probably take a look before merging :).
Mesa usually uses the timestamps of the llvm and driver shared libraries
as a cache key. In /nix/store these are all zero, so we'll include
$(drivers) in the cache key, which should be unique for all combinations
of mesa and llvm versions.