meson: Improve certifi documentation on macOS

First, just installing certifi doesn't install the ca-cert in the
right location. The `Install Certificates.command` script also
symlinks the openssl cert.pem to the certifi ca cert file

Second, we can make it more likely that users will notice this if we
make it a warning. If we ever get a bug report about this despite
these measures, we can try to make this an error.
25 jobs for !2707 with improve-certifi in 1 minute and 22 seconds (queued for 2 seconds)
merge request