Add rust-toolchain.toml

[xoloki]

Description

This PR adds a rust-toolchain.toml file to the rust directory. This file specifies the toolchain that should be used when building Rust code, so the developer will be able to switch the default toolchain (e.g. back to stable) and wallet-core will still build.

This was discovered as part of #4567

How to test

# switch the default toolchain
rustup default stable
# reconfigure 
./bootstrap.sh native
# rebuild
./tools/build-and-test

Types of changes

This PR doesn't change anything, it simply allows the repository to function for developers who have to switch toolchains.

• Bug fix (non-breaking change which fixes an issue)

Checklist

• Create pull request as draft initially, unless its complete.
• Add tests to cover changes as needed.
• Update documentation as needed.
• If there is a related Issue, mention it in the description.

If you're adding a new blockchain

• I have read the guidelines for adding a new blockchain.