Refactor Bonsai as workspace and use Foundry as external dependency

[nategraf]

Currently on main the Foundry tools, including forge are declared as build dependencies in bonsai/ethereum/Cargo.toml and so Foundry is built as part of building and testing the risc0 repo.
This turns out to be problematic, with the largest issue being dependency conflicts that keep breaking builds on different developer machines.
Foundry CLI is not meant to be imported as a Rust dependency, and it is becoming clear the that it should not be used as such.

This PR first refactors the build for bonsai/ethereum contracts to call forge as a subprocess rather than building and running it in-process.
If this is done alone, a consequence is that building risc0 will require installing the Foundry tools, even if the developer has no interest in Ethereum integration or Bonsai.
In order to address this, the bonsai directory is refactored as its own workspace.

[SchmErik]

not sure what's going on exactly but disabling the test seems to be causing CI to fail

[flaub]

I wonder if this rename is the culprit for stalled jobs