ci: add --mutex network to all yarn installs

Created by: ggilmore

We've made some recent improvements to CI so that it can run certain commands/scripts in parallel. Some of these parallelized scripts run yarn installs.

However, since then I've noticed some flaky yarn CI steps: https://buildkite.com/sourcegraph/sourcegraph/builds/50480#b996b624-73ef-4fa3-a0a3-6c5b96e700f4

This flakiness occurs because yarn's global cache isn't threadsafe. Using the yarn --mutex network flag ensures that only one yarn install can run at a time - which should fix this issue. We already used this flag in our docker image build scripts, but this PR adds this flag to all the yarn installs that CI runs.