Seems to be a Nix question really. https://nix.dev/manual/nix/2.24/language/builtins.html#builtins-getFlake
This function is only available if the flakes experimental feature is enabled. [...]
That's the thing, this is enabled and I can get a REPL with getFlake enabled system-wide, even with the Hydra user. Unfortunately, the hydra-evaluator doesn't seem to accept most Nix options, and I can't seem to figure out how to explicitly enable flakes for it.
$ hydra-evaluator --help
error: unrecognised flag '--help'
Try 'hydra-evaluator --help' for more information.
$ man hydra-evaluator
No manual entry for hydra-evaluator
Fernando Rodrigues