[fuchsia] Use correct label for the isolate

Change-Id: I00a1cd9c74f6f58b89e35068721dbc30abf396e8
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1955732Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Liviu Rau <liviurau@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65401}

infra/mb/gn_isolate_map.pyl

@@ -76,7 +76,7 @@
     "type": "script",
   },
   "fuchsia-unittests": {
-    "label": "//test/unittests:unittests",
+    "label": "//test/unittests:v8_unittests_fuchsia",
     "type": "script",
   },
   "webkit": {