[Toybox] mac CI breakage

[enh]

the mac build on github has been broken for a few days:

Run make
scripts/make.sh
readlink: illegal option -- f
usage: readlink [-n] [file ...]
readlink: illegal option -- f
usage: readlink [-n] [file ...]
$GENDIR=generated cannot include $***
make: *** [toybox] Error 1
Error: Process completed with exit code 2.

(from
https://github.com/landley/toybox/runs/6682393952?check_suite_focus=true)

funnily enough, it works fine for me locally because i'm on macOS 12, and
one of the few improvements i've actually noticed in it is that readlink
now has a -f option. i've attached a patch that switches the CI over to
just testing the current macOS release, but 10.15 is actually still getting
security updates (until the end of this year, if history is any guide).

note that it's possible to tell the compiler you're _targeting_ an older
macOS version, so we could do that if you don't care about _building_ on
old versions of macOS [without installing a custom readlink] but still want
to support _running_ on them...
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