quiche doesn't automatically configure a set of CA certificates to use
when enabling verification (as it doesn't enable verification by default
either), so it was never possible to connect to an endpoint with a valid
certificate, as the set of trusted root was empty.

Fix this, by calling
`qconfig.load_verify_locations_from_{directory_file}` with the results
of a `openssl_probe::probe()` call - the same that rustls-native-certs
uses in the Linux case.
It uses SSL_CERT_FILE / SSL_CERT_DIR env vars, with sane defaults.

I opted against using rustls-native-certs. It also supports reading from
the system certificate store / keychain on Windows and MacOS, but as quiche only
allows specifying a directory/file where it reads these on its own, and
not from memory, and writing this kind of stuff to a temporary file
before reading it back in sounds like a nightmare.

With this, I'm able to point psq-client to my server with a LE
certificate, without having to specify `--ignore-cert`.