Fix cargo-vet CI step by use stable rust toolchain

[gwen-lg]

It's not necessary to use the toolchain specified for puffin. And cargo-vet failed to build with 1.76 toolchain.

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

Setup cargo-vet CI step to build cargo-vet with stable Rust toolchain.

Related Issues

Currently cargo-vet v0.9.1 failed to build with Rust toolchain v1.73.0

error: failed to compile `cargo-vet v0.9.1`, intermediate artifacts can be found at `/tmp/cargo-installc9q6ok`.
To reuse those artifacts with a future compilation, set the environment variable `CARGO_TARGET_DIR` to that path.

Caused by:
  package `cargo-platform v0.1.9` cannot be built because it requires rustc 1.78 or newer, while the currently active rustc version is 1.76.0