This allows for cross-flake testing of content-addressible nix derivations without
affecting other users of the flake (which would be the cases if CA-drvs would be
hard-coded upstream) and without losing the ability to quickly update to the latest
idris2 version (which would happen if a fork of the repo would be used).
See also: https://discourse.nixos.org/t/content-addressed-nix-call-for-testers/12881
The flake can still be used as usual, but it will gain an output `overrideOptions`, which can be used like e.g.
```nix
let
idrisx = idris.overrideOptions {
config.contentAddressedByDefault = true;
};
in { ... }
```
