This requires upgrading to a newer version of `nixpkgs` to pick up
a newer version of the `tasty-bench` package in order to support
benchmarking using wall-clock time
The original reason for not using this was to
work around a mistake on our end. When we specified
`nix` via the `extra-libraries` clause of the `.cabal`
file instead of `pkgconfig-depends` that messed up the
file generated by `cabal2nix` which entailed various
work-arounds. Those workarounds are no longer necessary,
so we can switch to `packageSourceOverrides`
