Update to most recently released version. Also respect $NIX_BUILD_CORES now that the upstream build has a way to do so.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• No reverse dependencies
• Tested execution of all binary files (usually in ./result/bin/)
• No executables of interest
• Determined the impact on package closure size (by running nix path-info -S before and after)

/nix/store/fxi0ajklnlqqsk2sbjbf8w6ai6vmrw0z-python3.7-onnx-1.6.0          336450424
/nix/store/wls15nm3zgrw19pkw71asz2cp8gxla05-python3.7-onnx-1.7.0          340229832

• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.