Build scripts should not write outside OUT_DIR

[gmacon]

According to the Cargo book, build scripts should not modify any files outside of $OUT_DIR. The documentation here currently recommends that consumers of this crate do exactly that:

bindgen_cuda/src/lib.rs

Lines 297 to 304 in a6b0c89

	/// Consumes the builder and outputs 1 ptx file for each kernels
	/// found.
	/// This function returns [`Bindings`] which can then be unused
	/// to create a rust source file that will include those kernels.
	/// ```no_run
	/// let bindings = bindgen_cuda::Builder::default().build_ptx().unwrap();
	/// bindings.write("src/lib.rs").unwrap();
	/// ```

This caused an issue for me building candle-kernels within Nix because the source of candle-kernels is read-only when the build script is run.

It would be better for us to recommend the pattern given in the code generation example that writes to $OUT_DIR and then uses the include! macro in lib.rs.