Building ghc from source

[quantum9Innovation]

Describe the bug

Every haskell.nix setup I use seems to build GHC from source, even after correctly specifying the binary cache, using a supported GHC version, and pinning nixpkgs to a supported version used by haskell.nix.

• Build system x86_64-linux
• GHC version 9.12.2
• Haskell.nix version latest aa4b343

Steps To Reproduce

Following these instructions. Fails with both ghc96 and ghc912. Haven't tested other versions of GHC, but both of these should be supported and pulled from cache.

Expected behavior

GHC should be pulled from the IOHK cache and not built from source.

Additional context

I previously did not have this issue, but after clearing my machine's Nix cache, GHC is being built from source each time. This may be due to using a newer version of Haskell.nix; I will try downgrading to see if the old version works as expected.

[quantum9Innovation]

The issue seems to be a recent addition. I've bisected the commits and found this error was introduced in #2444 in commit 0928a5c.

From what I understand, this PR allows for prebuilt proprietary binaries to be included in the build process, but by default no such binaries should be used. Somehow this is causing Nix to build GHC from source—not sure why this is happening still, but could have to do with settings for dealing with unfree software.