Respect sandbox boundaries in pgx-examples

[Hoverbear]

After #461 I was testing https://github.com/zombodb/plrust and noticed this:

cargo-pgx-deps> [naersk] cargo_build_output_json (created): /build/tmp.3AkWFzqdBS
cargo-pgx-deps> [naersk] crate_sources: /nix/store/0fdvbq0jrl852dwpwldjfkcd6wzbk8d7-crates-io
cargo-pgx-deps> [naersk] RUSTFLAGS:
cargo-pgx-deps> [naersk] CARGO_BUILD_RUSTFLAGS:
cargo-pgx-deps> [naersk] CARGO_BUILD_RUSTFLAGS (updated): --remap-path-prefix /nix/store/0fdvbq0jrl852dwpwldjfkcd6wzbk8d7-crates-io=/sources
cargo-pgx-deps> building
cargo-pgx-deps> cargo build $cargo_release -j "$NIX_BUILD_CORES" --message-format=$cargo_message_format --package cargo-pgx
cargo-pgx-deps> error: failed to load manifest for workspace member `/build/dummy-src/pgx-examples/aggregate`
cargo-pgx-deps> Caused by:
cargo-pgx-deps>   failed to load manifest for dependency `pgx`
cargo-pgx-deps> Caused by:
cargo-pgx-deps>   failed to read `/build/pgx/pgx/Cargo.toml`
cargo-pgx-deps> Caused by:
cargo-pgx-deps>   No such file or directory (os error 2)
cargo-pgx-deps> [naersk] cargo returned with exit code 101, exiting
note: keeping build directory '/tmp/nix-build-cargo-pgx-deps-0.4.0-beta.0.drv-0'
error: builder for '/nix/store/q1qzc47d0kqxzc376aichykyc1xypy0i-cargo-pgx-deps-0.4.0-beta.0.drv' failed with exit code 101
error: 1 dependencies of derivation '/nix/store/l3v60hd25b3cjqsivg5ybn00cpwnjfcg-cargo-pgx-0.4.0-beta.0.drv' failed to build
error: 1 dependencies of derivation '/nix/store/klkjkp9cz216s7n9vl9ifscdkz7324jm-plrust-pg11-0.0.0.drv' failed to build

This was due to the pgx-examples stepping outside the sandbox (then back in):

https://github.com/zombodb/pgx/blob/9f2164fe248d59edd7782b27f9ad6afc28b7c418/pgx-examples/custom_types/Cargo.toml#L19