support overrides

[spiral-ladder]

we should allow vanilla fuelup installations (no toolchain installed), then only download the overridden toolchain/components with fuel-toolchain.toml detected in the project.

[spiral-ladder]

This is currently blocked due to fuelup requiring a default toolchain first: https://github.com/bingcicle/sway-sample/actions/runs/4013023554/jobs/6891948703