Speed up linting with better cache files

[samreid]

From a TODO in #1356:

  // TODO: https://github.com/phetsims/chipper/issues/1356 do we need a different cache for different repos or repo combos? We get a speed boost when using repos.join as a filename key
  args.push( '--cache', '--cache-location', '../chipper/eslint/cache/.eslintcache' );

We used to do this, I don't recall why we stopped. An experiment showed that using one mini-cache per repo gave a speed boost, especially when lint-everything is running one repo per process.

[samreid]

The benefit from this is negligible, closing.