Use cargo fetch for updating lock files

[rarkins]

Describe the proposed change(s).

Discussed in #29539

Per https://doc.rust-lang.org/cargo/commands/cargo-fetch.html, "cargo fetch" will update a lock file to match the Cargo.toml

This command would be better to use than our existing cargo update one. I think we can adopt this algorithm:

• If there are any non-lockfile-only updates (i.e. Cargo.toml is updated) then we run a single cargo fetch
• If there are any lockfile-only updates then we run cargo update targeting only those dependencies (similar to today)

We might need an additional cargo fetch after the cargo update commands, but we can leave that out for now if we're not sure.

[renovate-release]

🎉 This issue has been resolved in version 38.42.0 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 38.42.0

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