Use Treadle (on pull requests only) to speed up CI (backport #2341) (#2748)

* Support using Treadle for 'sbt test'

Treadle will be used as the "defaultBackend" when the environment
variable CHISEL3_CI_USE_TREADLE is set. The intent is to set this
variable during CI for pre-merge CI (aka on pull requests).

(cherry picked from commit 7d39b7bd)

* Use Treadle for CI on pull requests

(cherry picked from commit 82660673

)

Co-authored-by: Jack Koenig <koenig@sifive.com>