My build server which isn't using cache.nixos.org discovered an
outdated hash in servant:
```
trying https://github.com/haskell-servant/servant/archive/v0.16.2.tar.gz
% Total % Received % Xferd Average Speed Time Time Time Current
Dload Upload Total Spent Left Speed
100 132 0 132 0 0 616 0 --:--:-- --:--:-- --:--:-- 616
100 295k 0 295k 0 0 269k 0 --:--:-- 0:00:01 --:--:-- 384k
unpacking source archive /build/v0.16.2.tar.gz
hash mismatch in fixed-output derivation '/nix/store/i6qgxlqf599wl11rd44jasgmwb78wr6c-source':
wanted: sha256:0kqglih3rv12nmkzxvalhfaaafk4b2irvv9x5xmc48i1ns71y23l
got: sha256:0xk3czk3jhqjxhy0g8r2248m8yxgvmqhgn955k92z0h7p02lfs89
```
1cbb793ec8