[test] Use even less parallelism to prevent OOMs on ODROIDs

Apparently the first attempt wasn't working yet:
https://ci.chromium.org/p/v8/builders/luci.v8.ci/V8%20Arm/8806

TBR=sergiyb@chromium.org
NOTRY=true

Change-Id: I5864845e59508cd4cd1836798b56539d74bbb82e
Reviewed-on: https://chromium-review.googlesource.com/c/1335450Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#57490}

infra/testing/builders.pyl

@@ -1059,7 +1059,7 @@
           'cpu': 'armv7l-32-ODROID-XU4',
         },
         # Less parallelism to prevent OOMs in benchmarks.
-        'test_args': ['-j4'],
+        'test_args': ['-j2'],
       },
       {
         'name': 'optimize_for_size',