[fuchsia] Switch base_unittests to use archivist-for-embedding.cmx

Update base_unittests.cmx to inject archivist-for-embedding.cmx instead
of archivist.cmx, which has been removed in Fuchsia.

Bug: 1139075
Change-Id: Id5d30e5c9be7b14e9c5c7f4633ceb5ef4a032ca6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2477423
Auto-Submit: David Dorwin <ddorwin@chromium.org>
Commit-Queue: Sharon Yang <yangsharon@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Sharon Yang <yangsharon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#817724}