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}