I think we should make the following changes to cross_compile_assistance():

• Make it return a Result<Config, CrossCompileConfigError> instead of the three-state CrossCompileAssistance enum, where some of the states really should be errors. (The call site can still choose to ignore the errors if eg --no-cross-compile-assistance is used.)
• De-duplicate the help text
• Stop ignoring I/O errors that occur when using which (other than not found errors)

See:
https://github.com/heroku/libcnb.rs/blob/main/libcnb-package/src/cross_compile.rs