You can build .NET 8.0 from the repository by cloning the release tag v8.0.4 and following the build instructions in the main README.md.
Alternatively, you can build from the sources attached to this release directly.
More information on this process can be found in the dotnet/dotnet repository.
Attached are PGP signatures for the GitHub generated tarball and zipball. You can find the public key at https://dot.net/release-key-2023