fix: only generate linkage if CARGO_WORKSPACE_DIR is available

[schell]

RenderlingPaths

This makes the CARGO_WORKSPACE_DIR env var optional.

Without this env var we can't compile shaders or generate linkage.

Neither of those things should happen when cargo builds this library as a dependency, so this is fine.

Note that calling cargo xtask compile-shaders or cargo xtask generate-linkage will err if CARGO_WORKSPACE_DIR is not available, but the env var is always available inside renderling's source tree. Outside the source tree the xtask program won't be available, so the situation where xtask cannot build shaders or generate linkage is unreachable.

Dependencies

This also updates dependencies.

I've pinned wgpu to 23.0 instead of tracking main.

I've pinned spirv-std to a specific rev to fix glam mismatches.

Fixes #153.