chore: Fix build for downstream Cargo workspaces

[samuelburnham]

Fixes an issue where a downstream Lake project that contains a Cargo.toml workspace would error when trying to build the Blake3.lean dependency:

error: current package believes it's in a workspace when it's not:
current:   /path/to/downstream/.lake/packages/Blake3/rust/Cargo.toml
workspace: /path/to/downstream/Cargo.toml

this may be fixable by adding `.lake/packages/Blake3/rust` to the `workspace.members`
array of the manifest located at: /path/to/downstream/Cargo.toml
Alternatively, to keep it out of the workspace, add the package to the `workspace.exclude`
array, or add an empty `[workspace]` table to the package's manifest.

This is because the Blake3 Rust crate walks up the .lake directory tree until it finds a workspace, which it doesn't find when imported into non-workspace projects. This PR opts for the empty [workspace] solution so each consumer doesn't have to do the workaround.