Use environment variable OUT_DIR for the location of bindings.json

[vectronic]

I have a docs.rs build issue when the deno_bindgen attribute is encountered:

https://docs.rs/crate/flowscripter_template_deno_rust_library/1.0.6/builds/556217

From what I can tell this is because deno_bindgen is writing (always) the bindings.json file to the root of the project and this will fail on docs.rs because of readonly permissions:

https://docs.rs/about/builds#read-only-directories

The proposed fix for this is to write build output files into the cargo output directory defined in the environment variable OUT_DIR.