`fmt` panic on width argument

[tertsdiepraam]

There's a runtime error in fmt because OPT_WIDTH is not marked as a flag in clap:

thread 'main' panicked at 'Mismatch between definition and access of `width`. Could not downcast to bool, need to downcast to alloc::string::String
', /home/terts/Programming/uutils/coreutils/src/uu/fmt/src/fmt.rs:171:21

Originally posted by @tertsdiepraam in #4747 (comment)