CI fix

[lalitb]

Changes

CI is failing with error -

   Installed package `cargo-check-external-types v0.1.11` (executable `cargo-check-external-types`)
Running rustdoc to produce json doc output...
[/home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cargo-check-external-types-0.1.11/src/main.rs:105:30] err = Error {
    context: "error at /home/runner/.cargo/registry/src/index.crates.io-6f17d22bba15001f/cargo-check-external-types-0.1.11/src/main.rs:175:14",
    source: "The version of rustdoc being used produces JSON format version 29, but this tool requires format version 28. This can happen if the locally installed version of rustdoc doesn't match the rustdoc JSON types from the `rustdoc-types` crate.\n\nIf this occurs with the latest Rust nightly and the latest version of this tool, then this is a bug, and the tool needs to be upgraded to the latest format version.\n\nOtherwise, you'll need to determine a Rust nightly version that matches this tool's supported format version (or vice versa).",

Using the rust nightly release which uses the compatible rustdoc version as used by the rustdoc-type crate. It seems #1655 has somehow overwritten the default nightly version to latest.

Merge requirement checklist

• CONTRIBUTING guidelines followed
• Unit tests added/updated (if applicable)
• Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• Changes in public API reviewed (if applicable)