When compiling in GitHub Actions it's necessary to target x86_64-unknown-linux-musl, which currently can only be set using the CARGO_BUILD_TARGET environment variable - and even then, the runtime doesn't know to look in target/x86_64-unknown-linux-musl/... for build artifacts.
Detection of CARGO_BUILD_TARGET would probably be enough, idk if the runtime can be configured with CLI args or via vercel.json.