message: make TBB path ref more robust
diff: @@ -15,7 +15,7 @@ def runTests(String prefix = "") {
  }
  def runWinTests(String prefix = "") {
  - withEnv(['PATH+TBB=./stan/lib/stan_math/lib/tbb']) {
  + withEnv(['PATH+TBB=${env.WORKSPACE}/stan/lib/stan_math/lib/tbb']) {
  """ mingw32-make -j${env.PARALLEL} build
  ${prefix}runCmdStanTests.py -j${env.PARALLEL} src/test/interface
  """
  