Cache eslint on CI

[Dassderdie]

Store the info about processed files in order to only operate on the changed ones. The cache is stored in .eslintcache by default. Enabling this option can dramatically improve ESLint's running time by ensuring that only changed files are linted. https://eslint.org/docs/user-guide/command-line-interface#--cache

This would probably be done the same as with angular #32 - so the cache is updated after each run.