Support custom buildTypes?

[arianvp]

I'm trying to upload a SLSA predicate with a custom buildType

https://github.com/arianvp/nix-attest/blob/86781ad55047cefbe3ae8ea8fd2fab838aedc36b/hook.sh#L59
https://github.com/arianvp/nix-attest/blob/86781ad55047cefbe3ae8ea8fd2fab838aedc36b/.github/workflows/ci.yml#L68

However during upload I get an error:

https://github.com/arianvp/nix-attest/actions/runs/12681357992/job/35344912224

Error: Error: Failed to persist attestation: Invalid Argument - unsupported build type: https://nixos.org/build/v1 - https://docs.github.com/rest/repos/repos#create-an-attestation

however nowhere is documented what build types are supported. Furthermore It would be great if custom build types are allowed just like custom predicate types