For v6, don't use local ember-cli in place of the invoked ember-cli

This has always been the behavior, but I've always found it really annoying.

specifically when I want to make new projects in an existing repo, I use npx ember-cli@latest, but if an older ember-cli is found anywhere that older one is used instead (even if the package.json doesn't specify it).

We should remove this behavior, or disable it by default.