point hhvm stacktraces to workspace log dir

The mediawiki-core-phpunit-hhvm job fill up /tmp with stacktrace files
generated whenever a backtrace is emited.  The generate file is neither
cleaned up nor archived with the build.

From hphp/runtime/bas/crash-reporter.cpp:

  char tracefn[RuntimeOption::CoreDumpReportDirectory.length()
               + strlen("/stacktrace..log") + strlen(pid) + 1];

Pointing Debug.CoreDumpReportDirectory to the workspace log dir should
let us compress and capture the trace.

Bug: 62787
Change-Id: I431d5868d5eea2ce8c361241df5a3b16b152b10f