chore: update vendored yarn

chore: update vendored yarn

Done using:

    wget https://yarnpkg.com/latest.tar.gz
    gunzip latest.tar.gz
    tar xf latest.tar
    rm latest.tar
    git rm -r yarn-v1.22.4
    mv yarn-v1.22.5 yarn
    git add yarn
    git staged # eyeball changes...

Note that I renamed the install directory from "yarn-$VERSION" to plain
old "yarn" to keep things looking clean. In the end seeing the version
at a glance by looking in the filesystem isn't super useful; when it's
time to update, Yarn will nag anyway.