Use Travis Workspaces (#1675) (#1677)

* Use Travis Workspaces

Add stage to build FIRRTL and share it with later steps running the
tests

* [CI] Do not sbt clean in formal_equiv.sh

This change takes advantage of shared workspace in Travis

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 1769f8d7

)

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