Summary:
Previously, we were running the `eslint_ui.sh` helper script as part of the version check even if we didn't have any files to lint with `eslint`.
To speed it up, I removed the version check.

However arcanist runs linters in parallel, which now meant that we had multiple `yarn install`s running concurrently.
These would stomp on the contents of `node_modules` and the `eslint` binary causing some lint runs to emit no output which in turn cause `arc` to fatal.

Instead we hook into `willLintPaths` and run a setup script once if there are any paths before calling the parent class's `willLintPaths` which will actually create the futures to run the linter.

Test Plan:
Before this change
- edit multiple `ts` files
- `rm -rf src/ui/node_modules; arc lint --trace`
- notice failure

After this change
- edit multiple `ts` files
- `rm -rf src/ui/node_modules; arc lint --trace`
- notice `eslint_setup.sh` being run first
- notice `eslint_ui.sh` being run in...