Fix docs.rs build

The [docs.rs build](https://docs.rs/crate/pdal/0.1.0/builds/1187387) is failing due to PDAL not being available in the documentation environment.

Advice has [been requested](https://github.com/dtolnay/cxx/issues/1341) from the `cxx` community to determine what the best path forward is.

## Notes

See also: https://docs.rs/about/builds

### Building in docs.rs environment

1. `git clone https://github.com/rust-lang/docs.rs.git`
2. `cd docs.rs`
3. Edit `docker-compose.yaml` and add a volume mount to your package (figuratively):
`services.web.volumes[-1]: "/my/source/pdal:/opt/docsrs/pdal:ro"`
4. `docker-compose up -d db s3`
5. Run build: `docker-compose run web build crate --local /opt/docsrs/pdal`
6. View build: `cargo run -- start-web-server`