Add fromToolchainName to targets

[soywod]

This would allow combining components from a specific version in a cross compilation context:

[toolchain]
channel = "1.77.0"
profile = "default"
components = [ "rust-src", "rust-analyzer" ]

let
  name = (pkgs.lib.importTOML ./rust-toolchain.toml).toolchain.channel;
  fenixPackage = fenix.packages.${buildPlatform};
  toolchain = fenixPackage.fromToolchainName { inherit name sha256; };
  targetToolchain = fenixPackage.targets.${targetPlatform}.fromToolchainName { inherit name sha256; };
in
fenixPackage.combine [
  toolchain.rustc
  toolchain.cargo
  targetToolchain.rust-std
];