crates.io index is checked even for --no-push to a private registry

[bobrik]

If a public registry is not reachable, cargo release -x --no-push to a private registry cannot proceed.

One concrete example is MITM where extra CA is added to the developer machine for enterprisey reasons:

$ RUST_LOG=debug ~/projects/cargo-release/target/debug/cargo-release release -x --verbose --no-push 2023.12.1
[2023-12-18T22:24:06Z DEBUG reqwest::connect] starting new connection: https://index.crates.io/
[2023-12-18T22:24:06Z DEBUG rustls::client::hs] No cached session for DnsName("index.crates.io")
[2023-12-18T22:24:06Z DEBUG rustls::client::hs] Not resuming any session
[2023-12-18T22:24:06Z DEBUG rustls::client::hs] Using ciphersuite TLS13_AES_256_GCM_SHA384
[2023-12-18T22:24:06Z DEBUG rustls::client::tls13] Not resuming
[2023-12-18T22:24:06Z DEBUG rustls::client::tls13] TLS1.3 encrypted extensions: [ServerNameAck, Protocols([ProtocolName(6832)])]
[2023-12-18T22:24:06Z DEBUG rustls::client::hs] ALPN protocol is Some(b"h2")
error: error sending request for url (https://index.crates.io/th/re/thread-saturation-exporter): error trying to connect: invalid peer certificate: UnknownIssuer: error trying to connect: invalid peer certificate: UnknownIssuer: invalid peer certificate: UnknownIssuer

There is no easy way to tell rustls to use an extra CA here, but that's a little beside the point.

One can add a panic!() to RemoteIndex::krate to get the offending stack trace:

thread 'main' panicked at src/ops/index.rs:86:9:
oh no
stack backtrace:
   0: rust_begin_unwind
             at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/std/src/panicking.rs:597:5
   1: core::panicking::panic_fmt
             at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/panicking.rs:72:14
   2: cargo_release::ops::index::RemoteIndex::krate
             at /Users/ivan/projects/cargo-release/src/ops/index.rs:86:9
   3: cargo_release::ops::index::CratesIoIndex::krate
             at /Users/ivan/projects/cargo-release/src/ops/index.rs:50:21
   4: cargo_release::ops::index::CratesIoIndex::has_krate
             at /Users/ivan/projects/cargo-release/src/ops/index.rs:21:12
   5: cargo_release::steps::release::ReleaseStep::run
             at /Users/ivan/projects/cargo-release/src/steps/release.rs:76:16
   6: cargo_release::run
             at /Users/ivan/projects/cargo-release/src/bin/cargo-release.rs:30:17
   7: cargo_release::main
             at /Users/ivan/projects/cargo-release/src/bin/cargo-release.rs:9:15
   8: core::ops::function::FnOnce::call_once
             at /rustc/79e9716c980570bfd1f666e3b16ac583f0168962/library/core/src/ops/function.rs:250:5
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.

This is the place where it's called:

• https://github.com/crate-ci/cargo-release/blob/v0.25.0/src/steps/release.rs#L76

It doesn't feel right to check public registry to see whether a crate exists there when I'm trying to make a release for a private registry. Removing this check allows my releases to proceed as expected.

I'm not sure what's the right solution is here, but I'm certain that it should not require public registry queries.