yarn incorrectly uses $XDG_DATA_HOME for linkFolder

[asottile-sentry]

yarn's default for linkFolder is in the default for $XDG_CONFIG_HOME:

$ yarn config current | grep linkFolder
  "linkFolder": "/Users/asottile/.config/yarn/link",

the default for XDG_CONFIG_HOME is ~/.config

applying XDG_DATA_HOME changes the value of linkFolder even if XDG_DATA_HOME is set to the default:

$ XDG_DATA_HOME=$HOME/.local/share yarn config current | grep linkFolder
  "linkFolder": "/Users/asottile/.local/share/yarn/link",

this appears to be two bugs -- fixing either would resolve this:

• yarn/src/constants.js

  Line 63 in 158d96d

  export const LINK_REGISTRY_DIRECTORY = path.join(DATA_DIRECTORY, 'link');

  (this is using the data home as the joined value for linkFolder)

• yarn/src/util/user-dirs.js

  Line 22 in 158d96d

  return FALLBACK_CONFIG_DIR;

  getDataDir falls back to ~/.config