Add yarn-cache to ignored paths

[hugomrdias]

for some CIs we need to change yarn-cache to the working directory so it can be cache between builds

[marionebl]

The default yarn cache does not reside in process.cwd(), correct?
xo has to be conservative with additional default ignores to avoid surprises for users, so I'd vote against adding this special case.

You can ignore the directory via the ignore flag instead:

xo --ignore yarn-cache/**/*

or the ignores option:

{
  "xo": {
    "ignores": ["yarn-cache/**/*"]
   }
}

Alternatively you can add it to .gitignore as xo evaluates it, too.