The mystery build failure was caused by having the same instance as an
orphan and imported from ref-tf 0.5 (why ever that doesn't warrant a
logged error message…). The solution for this is
https://github.com/haskell-nix/hnix/pull/918, which sadly doesn't apply
cleanly on the hnix 0.12.0.1 tarball. Therefore I've backported the
patch until hnix hopefully gets a new hackage release soon.
