Fix support for Yarn 2 (berry)

[ylemkimon]

• Checks the version of Yarn installed globally using YARN_IGNORE_PATH=1. The environment variable YARN_VERSION sets the version of Yarn to install globally.
• Sets YARN_GLOBAL_FOLDER environment variable to the cache directory. The cache-folder flag is deprecated in Yarn 2. The user can opt-in to use the global cache by setting YARN_ENABLE_GLOBAL_CACHE in Netlify settings. (See https://github.com/netlify/build-image/pull/465#discussion_r465539682)

Closes #319.

Suggestion for Docs: https://docs.netlify.com/configure-builds/environment-variables/#netlify-configuration-variables

- * `YARN_VERSION`: Sets the [Yarn version].
+ * `YARN_VERSION`: Sets the [Yarn version] to install globally.
+ * `YARN_ENABLE_GLOBAL_CACHE`: Use the [global cache](https://yarnpkg.com/configuration/yarnrc#enableGlobalCache) for Yarn 2. 

https://docs.netlify.com/configure-builds/manage-dependencies/#yarn

- * `YARN_VERSION`: defaults to the version preinstalled with your initial [build image]. Accepts any released version number.
+ * `YARN_VERSION`: defaults to the version preinstalled with your initial [build image]. Accepts any released version number. The specified version is installed globally.
+ * `YARN_ENABLE_GLOBAL_CACHE`: whether to use the [global cache](https://yarnpkg.com/configuration/yarnrc#enableGlobalCache) for Yarn 2. If you use zero-installs, do not set this variable.

CC @arcanis Could you check whether this is the right way to use Yarn 2 in CI?

[benfurber]

Would love for this to be merged in so that I move a site over to netlify. 🙏

[kevinji]

The line below export PATH=$(yarn bin):$PATH also needs to be updated; yarn bin on Yarn 2+ no longer returns a node_modules/.bin directory (which doesn't exist with PnP) but instead just lists all available binaries.

[kitop]

Closing this PR as the it's against xenial branch and the Xenial image is going to be deprecated: https://answers.netlify.com/t/please-read-end-of-support-for-xenial-build-image-everything-you-need-to-know/68239

Feel free to reopen against focal branch.