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.