[benchmark-dtrace] Set SWIFT_DETERMINISTIC_HASHING=1 before calling subjobs.

This prevents a bunch of instability in the retain, release numbers. I am still
getting some of it, but this helps a lot.

Author: gottesmm

benchmark/scripts/Benchmark_DTrace.in

@@ -86,11 +86,13 @@ class DTraceBenchmarkDriver(perf_test_driver.BenchmarkDriver):
         sys.stdout.flush()
 
         def get_results_with_iters(iters):
+            e = os.environ
+            e['SWIFT_DETERMINISTIC_HASHING'] = '1'
             p = subprocess.Popen([
                 'sudo', 'dtrace', '-s', DTRACE_PATH,
                 '-c', '%s %s %s' % (data['path'], data['test_name'],
                                     '--num-iters=%d' % iters)
-            ], stdout=subprocess.PIPE, stderr=open('/dev/null', 'w'))
+            ], stdout=subprocess.PIPE, stderr=open('/dev/null', 'w'), env=e)
             results = [x for x in p.communicate()[0].split("\n") if len(x) > 0]
             return [
                 x.split(',')[1] for x in