This fixes using inputDerivation on derivations that are fixed-output.
Previously:
```
nix-repl> drv = runCommand "huh" { outputHash = "sha256-47DEQpj8HBSa+/TImW+5JCeuQeRkm5NMpJWZG3hSuFU="; outputHashAlgo = "sha256"; outputHashType = "flat"; } "touch $out"
nix-repl> drv.inputDerivation
«derivation /nix/store/d8mjs6cmmvsr1fv7psm6imis5pmh9bcs-huh.drv»
nix-repl> :b drv.inputDerivation
error: fixed output derivation 'huh' is not allowed to refer to other store paths.
You may need to use the 'unsafeDiscardReferences' derivation attribute, see the manual for more details.
```
Fixes: https://github.com/NixOS/nixpkgs/issues/304209
78945a827c