Fixup lib_name when using PYO3_CONFIG_FILE

[messense]

This matches the logic in default_cross_compile:

pyo3/pyo3-build-config/src/impl_.rs

Lines 1415 to 1426 in eafbbc5

	let lib_name = if cross_compile_config.target.operating_system == OperatingSystem::Windows {
	Some(default_lib_name_windows(
	version,
	implementation,
	abi3,
	false,
	))
	} else if is_linking_libpython_for_target(&cross_compile_config.target) {
	Some(default_lib_name_unix(version, implementation, None))
	} else {
	None
	};