Trigger errors in benchmarking Nix

[Chris Warburton]

When nix-eval needs some Haskell packages, it checks to see if they're
available. If so, it calls GHC as-is, if not it uses Nix to generate an
environment with those packages, potentially compiling them.

We don't want to be compiling during a benchmark, since it'll skew our
runtimes massively upwards. We *should* be running without having to
compile, since we're only using a given package (e.g. TEBenchmark,
list-full, etc.), but we can't be sure.

We should set the Haskell overrides used by nix-eval to some sort of
error, e.g. `abort "Benchmarking shouldn't need to make a new
env"`. This way, we'll know if there's a potential skew.