Test yarn installation on build

[tcr]

We found our first breaking change in #1030 with how yarn installs binaries.

I found the invocation to test installing a yarn module globally with a local t2-cli checkout:

yarn cache clean
yarn global add file:$(pwd)

First we clean the cache, otherwise a previously downloaded [email protected] might be selected instead of our local copy. Then we do a global add with the file: protocol of the current directory (t2-cli).