Don't require unneeded data when running icu4x-datagen without the networking feature

[sffc]

This should work, but it doesn't:

cargo run -p icu4x-datagen \
	--no-default-features \
	--features provider,fs_exporter \
	-- \
	--format fs \
	--pretty \
	-o _debug/data \
	--cldr-root provider/source/tests/data/cldr \
	--deduplication none \
	--locales ru \
	--markers DatetimePatternsDateGregorianV1

It only works after also setting --icuexport-root, --segmenter-lstm-root, and now --tzdb-root.

CC @robertbastian