Test local flow definition installs

[Brianzchen]

CLI Version

3.x.x

What problem do you want to solve?

From the recent change 9a968c6 I wasn't able to test the definitions until I merged the changes into master. This can cause unknown regressions

Your take on the correct solution?

In a local project we can normally run node ../flow-typed/cli/dist/cli.js install to validate that the CLI is operating as expected but this will still fetch definitions from github remote servers.

We could add some optional local env var so that when present if we ran the above command it would install the definitions locally.

Anything else?

This might also satisfy another requirement if someone ever wanted to install definitions from another server instead of github.com/flow-typed/flow-typed as they can define their other pathing.

Do you want to submit a pull request to implement this change?

Yes