The exact same problem exists with the channel that you acquire the public key you trust from. You’re still fundamentally trusting HTTPS to the package provider - you’re just trusting it at a different point.
Usually keyring is separate package which is also signed with a key which can be verified from multiple different sources.
Of course, if you are a target of nation state attack, which fakes public keys from all sources by MITMn DNSs and servers, you might end up with the wrong package.
> , without any third party verification.
Certificate provider does not verify packages nor anything what is coming from there. Server even might be just proxy.