Include .crates.toml in Rust example

Description

Using the existing example, cargo install will error out after restoring a cache:

cargo install cargo-tarpaulin
    Updating crates.io index
error: binary `cargo-tarpaulin` already exists in destination
Add --force to overwrite
Error: Process completed with exit code 101.

--force is undesirable because it will recompile the binary. Cargo is scared of the existing binary because it doesn't have enough information about it. Including ~/.cargo/.crates.toml coaxes it into being okay with the situation.

Motivation and Context

Caching cargo install PACKAGE errors with

error: binary `PACKAGE` already exists in destination

This can be circumvented by using --force but this negates any benefit of using the cache.

How Has This Been Tested?

I am using the example with this change on my own private repo successfully.

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[x] Documentation (add or update README or docs)

Checklist:

[x] My code follows the code style of this project.
[x] My change requires a change to the documentation.
[x] I have updated the documentation accordingly.
[x] I have read the CONTRIBUTING document.
[x] I have added tests to cover my changes.
[x] All new and existing tests passed.

actions / cache