Startup script: Use the CI rust version of polkadot-sdk

[kianenigma]

The getting started script introduced in #4879 is great, but it installs the latest rust verison by default, and this sometimes yields to warnings when compiling polkadot-sdk. It would be great to use the version that is expected to pass the CI.

This is visible in https://github.com/paritytech/polkadot-sdk/blob/master/.github/env, but perhaps it can be stored somewhere a bit more visible.

[Wolfenheimm]

Hello @kianenigma - I've come up with a solution that fetches the version directly from the file using grep/cut. Please let me know if perhaps I misunderstood or if you would like more functionality :)

PR: #5317

[alvicsam]

Another similar issue has been opened recently. We'll add rust-toolchain.toml with rust version as soon as we finish ci migration to github actions