Nix develop fails due to missing godot-haskell revision

[RedEtherbloom]

When trying to enter the Simula VR dev shell on the master branch, using nix develop it fails with this error message:

warning: ignoring untrusted flake configuration setting 'extra-substituters', pass '--accept-flake-config' to trust it (may allow the flake to gain root, see the nix.conf manual page)
warning: ignoring untrusted flake configuration setting 'extra-trusted-public-keys', pass '--accept-flake-config' to trust it (may allow the flake to gain root, see the nix.conf manual page)
error:
       … while evaluating an expression to select 'drvPath' on it
         at «internal»:1:552:
       … while evaluating strict
         at «internal»:1:552:
       (stack trace truncated; use '--show-trace' to show the full trace)

       error: Cannot find Git revision 'b06876dcd2add327778aea03ba81751a60849cc8' in ref 'refs/heads/master' of repository 'https://github.com/SimulaVR/godot-haskell'! Please make sure that the rev exists on the ref you've specified or add allRefs = true; to fetchGit.

I haven't tested any other branches yet. I'll assume that extra-substitures shouldn't make a difference for finding the missing rev.
I have cloned the repo, including it's submodules, with the recursive flag as stated in the Readme.