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

it uses bash, at least.