Before, the workspace root was compiled into the binary. Since our
tests depend on the GerritLauncher, this killed any kind of cross-user
test caching.
For Polygerrit development, run gerrit as
java -DsourceRoot=/path/to/my/checkout -jar gerrit.war
Change-Id: I3321f26dafeb68bfa924c53b686d066a921803d0