See also #353.
In the internal build environment, there's an auto-injected component governance task which needs to run once during the build. This task doesn't need to run during the unit test, package, and internal release jobs.
* Move build jobs to the Hosted VS2017 pool instead of our internal pool
* Move the prepare-release-internalonly job from a team-specific pool to a shared pool with more capacity
* Remove symbol publishing from PR/CI builds since it's not necessary; do it only in release builds
