darcs is broken on master

[srhb]

Issue description

Missing dependency: graphviz >=2999.18.1 && <2999.20

Jailbreaking darcs leads to

Warning:
    This package indirectly depends on multiple versions of the same package. This is very likely to cause a compile failure.
      package graphviz (graphviz-2999.20.0.2-JOtutOW3cVZKoS3BCpJuAB) requires temporary-1.2.1.1-9BOgWDdFKJ63zHv30zIEQK
      package darcs (darcs-2.14.0) requires temporary-1.2.1.1-9NvLhs3zcHhBObXNI86Qp9

Steps to reproduce

nix-build '<nixpkgs>' -A darcs

Technical details

nixpkgs rev 288ce0c