How do I specify the floating-point abi when building a flake?

The systems used in flakes are all doubles, e.g. “armv7l-linux”. How do I specify if I want hard or soft float?
I’ve gathered from reading the source code that if unspecified, the default is hard float. I have some existing (proprietary) libraries using the soft floating-point ABI, and I’d like to build software with Nix flakes that can interoperate with them. Is it possible to do this?