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 Fix support for Yarn 2 (berry) #465 (comment))

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?

[arcanis]

Thanks for doing this! I've left a few comments, I hope someone from Netlify will be able to take a look.

[arcanis]

I'm not a fan of using environment variables for this, because it'll break people using zero-installs (when the cache is inside the repository), overriding the config they explicitly requested (I know it's what the code already did, overriding --cache-folder etc, but that really strikes me as a bad idea).

I think a more gentle approach would be to create a .yarnrc.yml file in the home directory that would contain the "default options" you want to have. This way, users would still be able to opt-out of the default workflow to use their own.

[ylemkimon]

@arcanis Thank you for the review! I've removed export YARN_ENABLE_GLOBAL_CACHE=1. The user now can opt-in to use global cache by setting YARN_ENABLE_GLOBAL_CACHE in Netlify settings.

I guess it'd be for Netlify to decide the default for enableGlobalCache and whether to allow users to override the global folder, so I'll leave it for now.

[kara-todd]

Chiming in here... :)

I've also attempted to use zero-installs on Netlify and ran into issues as the install command always seems to run. Would it be possible to amend these changes to allow skipping of the install step completely? Or did I just miss how to do that 🙃 ?

[ylemkimon]

@kara-todd AFAIK, shoudn't yarn install only take few seconds with zero-installs? You can set YARN_ENABLE_NETWORK=0 to disallow making network requests.

[kara-todd]

@ylemkimon when I was running it... it still seemed to still run the relink step, but I didn't try it with YARN_ENABLE_NETWORK=0. It didn't take terribly long as I recall... 🤔 However, I use this in conjunction with Netlify CMS which has to wait on builds for content reviews... So every second ends up being precious. :)

I know that part of the problem was caused by Netlify not correctly detecting the cache directory as you are trying to fix here. Still, a true zero install where I only run my build command would be preferable to me if it's not a huge lift.

[ylemkimon]

@kara-todd I have a Netlify repo with zero-install and it takes less than two seconds running yarn install. If it takes more than a few seconds, the repo may not have been set up properly. A workaround for now would be move the whole project into a subdirectory, not having package.json and yarn.lock at the root, and cd in the build command.

[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.