When the nixpkgs revision was bumped, the stack version in use was bumped.
The newer stack version uses a different cache layout and can't find any of
the cached artifacts in the old cache.

???

Okay what is it

`stack path --fast ...` says "Invalid option `--fast'"

From #haskell:

> that might change optimization and thereby the abi since cross-module
> inlining information will be absent with -O0 / no -O

privacypass Nix packaging is sensitive to the version and the one previously
selected is now too old

try to get around the timeout problem on slow CircleCI builders