CI tests are working now.

For posterity, here's my reasoning for doing this change:

• Most cases don't actually spawn local binaries from node_modules.
  I realized that most of the cases are global binaries and local ones with {cwd: __dirname}.
• The few times you actually need it, it might be more readable to make it explicit.
• It causes surprising behavior then because:
  1. Yarn mocks with the PATH too.
  2. Some Node.js installation has a node_modules directory or npm directory that will be used instead of what the user actually wanted. This can happen silently and cause confusion.

So basically, I think we should default to the least surprising behavior.