Published async-channel 1.7.0 failed to compile on MSRV

[taiki-e]

https://github.com/smol-rs/async-fs/runs/7738662148?check_suite_focus=true

error: failed to parse manifest at `/home/runner/.cargo/registry/src/github.com-1ecc6299db9ec823/async-channel-1.7.0/Cargo.toml`

Caused by:
  feature `resolver` is required

  this Cargo does not support nightly features, but if you
  switch to nightly channel you can add
  `cargo-features = ["resolver"]` to enable this feature

This is due to a cargo bug: rust-lang/cargo#10954

[taiki-e]

Fixed in 1.7.1 by publishing with stable cargo.