spirv-builder should declare dependencies on Cargo.toml files.

For #403 I changed the workspace Cargo.toml, but that didn't result in a rebuild of sky-shader, whereas something like this does cause a rebuild (of example-runner-wgpu, in this example):

[profile.dev.package."example-runner-wgpu"]
opt-level = 3

So I think we need spirv-builder to get the workspace Cargo.toml (and Cargo.lock, I suppose) paths from Cargo somehow, and also list those out as "rebuild if changed".