Auto merge of #8571 - canova:publish-default, r=ehuss

Auto merge of #8571 - canova:publish-default, r=ehuss

Default cargo publish to the alt registry if it's the only allowed one

Hi, this PR fixes #8036. Previously if we had only one allowed registry in Cargo.toml file, `cargo publish` was failing. But with this PR, we are defaulting to the only allowed registry with printing a note if there is only one registry in that array. I believe this will make things easier for people who use an alternative registry all the time.