Make rustc --explain compatible with BusyBox less

[omnivagant]

busybox less does not support the -r flag and less(1) says:

USE OF THE -r OPTION IS NOT RECOMMENDED.

[rustbot]

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[nnethercote]

USE OF THE -r OPTION IS NOT RECOMMENDED.

I see this text in the man page for GNU less, which is used on my Linux machine (unsurprising) and on my Mac laptop (huh?)

less is used as the pager on non-Windows sytems if PAGER isn't set. I guess the question is whether any versions of less (other than busybox's) have -R/-r that don't mean the same things as the GNU version.

https://man.freebsd.org/cgi/man.cgi?query=less shows that freebsd less is also the GNU one. Maybe that explains my Mac result above? Similar story with openbsd: https://man.openbsd.org/less.1. And netbsd: https://man.netbsd.org/less.1.

So, changing to -R seems ok for a lot of the popular non-Windows platforms, and fixes busybox, but I'm worried that I might be overlooking some other platform. Any suggestions how to overcome that worry?