Support yarn berry

[skaindl]

When using yarn berry (yarn 2.x), the yarn install command fails because yarn reserves all environment variables prefixed with YARN_ for its own configuration:

---> Using 'yarn install' with YARN_ARGS
(node:293) ExperimentalWarning: The dns.promises API is experimental
(node:293) ExperimentalWarning: The fs.promises API is experimental
Usage Error: Unrecognized or legacy configuration settings found: enabled - run "yarn config -v" to see the list of settings supported in Yarn (in <environment>)

$ yarn install [--json] [--immutable] [--immutable-cache] [--check-cache] [--inline-builds]
subprocess exited with status 1

So having the environment variable YARN_ENABLED, YARN_BUILD, or YARN_ARGS specified causes the build to fail. Please add a custom prefix all environment variables that begin with YARN_.