Don't consume newer version of Rust via `rustup update stable`

[jpmcb]

In the following GitHub workflow for CI builds:

https://github.com/bottlerocket-os/bottlerocket-update-operator/blob/047357262b02b462007451922844fb604c1566d4/.github/workflows/ci.yml#L30

we are consuming the newest version of rust "locally" which pulls down dependencies that get fed into the docker container that uses the bottlerocket sdk. This leads to the possibility of the "local" version of rust fast-forwarding from the version in the sdk. For example, at the time of this writing, the sdk is on 1.69, and in these breaking CI builds, rustup consumes 1.70:

https://github.com/bottlerocket-os/bottlerocket-update-operator/actions/runs/5189657965/jobs/9355968232?pr=350

We may be able to forgo doing the whole operation of getting rust "locally" and instead remove the --network=none line in the docker container so it can just do it all from within the build.