Set the workspace root through a JVM property sourceRoot

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

tools/workspace-status.sh

@@ -19,4 +19,3 @@ for p in plugins/* ; do
   test -d "$p" || continue
   echo STABLE_BUILD_$(echo $(basename $p)_LABEL|tr '[a-z]' '[A-Z]' ) $(rev $p)
 done
-echo "STABLE_WORKSPACE_ROOT ${PWD}"