Hey, it seems the env variable also works for me.
Still wanted to share the error I get, that was due to having a custom yarn install inside a node module (funny, because it was brought in the app from netlify-cli :D)
yarn why yarn
What I did is to force the node module to use the version I want instead, so that netlify does not use that older version.
“resolutions”: {
“yarn”: “1.21.1”
},