feat: keep backwards compatibility with `SSL_CERT_FILE` without requiring `--native-tls` (#2401)
## Summary
Small follow up to https://github.com/astral-sh/uv/pull/2362 to check if
`SSL_CERT_FILE` is set to enable `--native-tls` functionality. This
maintains backwards compatibility with `0.1.17` and below users
leveraging only `SSL_CERT_FILE`.
Closes https://github.com/astral-sh/uv/issues/2400
## Test Plan
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Assuming `SSL_CERT_FILE` is already working via `--native-tls`, this is
simply a shortcut to enable `--native-tls` functionality implicitly
while still being able to let `rustls-native-certs` handle the loading
of `SSL_CERT_FILE` instead of ourselves.
Edit: Manually tested by setting up own self-signed CA certificate
bundle and set `SSL_CERT_FILE` to this and confirmed the loading happens
without having to specify `--native-tls`.