Motivation for this change

Setting a Bundler version with GEM_PATH doesn't seem to work in Ruby
2.7, so we need to use the LOAD_PATH instead. Without this,
bundlerEnv environments will always use the version of Bundler that
comes with Ruby, which won't necessarily work because it isn't the
version that was used to generate the bundle.

For example, building ronn with Ruby 2.7 without this change results
in a broken executable, but it works (when built with all packaged
Ruby versions) after 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"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

Notify maintainers

cc @