[test] Add fake command to swarming isolate

Temporary fix for isolate archiver behaving weirdly when the command is
missing.

NOTRY=true
TBR=sergiyb@chromium.org

Bug: v8:7012
Change-Id: I1ccee6be16605426e74b6c4a0379fa502106e5a9
Reviewed-on: https://chromium-review.googlesource.com/768727Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49344}

tools/run-num-fuzzer.isolate

@@ -3,6 +3,9 @@
 # found in the LICENSE file.
 {
   'variables': {
+    'command': [
+      'run-deopt-fuzzer.py',
+    ],
     'files': [
       'run-deopt-fuzzer.py',
       'run-gc-fuzzer.py',