Install yarn<3 in dev env

[davidbrochart]

With yarn v3.6.0, installation fails with:

# This file contains the result of Yarn building a package (@voila-dashboards/voila-root@workspace:.)
# Script name: install

lerna notice cli v4.0.0
lerna info versioning independent
lerna info bootstrap root only
Unknown Syntax Error: Unsupported option name ("--mutex").

$ yarn install [--json] [--immutable] [--immutable-cache] [--check-cache] [--inline-builds] [--mode #0]
lerna ERR! yarn install --mutex network:42424 --non-interactive exited 1 in '@voila-dashboards/voila-root'
lerna ERR! yarn install --mutex network:42424 --non-interactive exited 1 in '@voila-dashboards/voila-root'

[github-actions]

👈 Launch a Binder on branch davidbrochart/voila/yarn

[trungleduc]

Thanks @davidbrochart!