Allow overriding paths to browsers #363

I'm on NixOS, so the browsers aren't in /opt/whatever and the root file system is read-only, so playwright-cli install could never work.

I would like to have an environment variable that can be used to set the path to the path where nix installed the playwright browsers, similarly to what is available for the playwright js lib.

I can currently use these instructions to successfully setup playwright scripts in an ordinary node environment, but that doesn't work with playwright-cli, since it seems to use hard-coded /opt/google/chrome/chrome paths, ignoring the PLAYWRIGHT_BROWSERS_PATH environment variable.