Auto merge of #6525 - ehuss:publish-crates-io-alt, r=nrc

publish: rework the crates.io detection logic.

The old code used an unreliable method to detect if `cargo publish` was publishing to crates.io. This should work even if the `--index` flag is used.

Also includes a very minor rewording of an error message mentioning crates.io, even for alternative registries.