Enable running pos tests in partest

As junit tests are run with dotty-compiled-by-scalac,
it makes sense to execute run tests with dotty-compiled-by-dotty.

Author: DarkDimius

scripts/jobs/validate/partest

@@ -4,5 +4,5 @@ baseDir=${WORKSPACE-`pwd`}
 scriptsDir="$baseDir/scripts"
 . $scriptsDir/common
 
-sbt $sbtArgs update compile "partest-only run --show-diff --verbose"
+sbt $sbtArgs update compile "partest-only --show-diff --verbose"