[roll] Roll fuchsia [assembly] Assert assembly is only run in the default toolchain

Assembly can only be performed in the default toolchain, so assert
that fact.

Original-Bug: b/377356462
Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1178012
Original-Revision: e8e2fa3297aedd25081ccc3c269ad07d9d50a07e
GitOrigin-RevId: 815c4954a497c4afa96a745e5eaec958853bd40d
Change-Id: Id76938259a451f43e0ed8933aba06be192414009
diff --git a/stem b/stem
index 1e700f8..9de4274 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
              githooks="integration/git-hooks"
              remote="https://fuchsia.googlesource.com/fuchsia"
              gerrithost="https://fuchsia-review.googlesource.com"
-             revision="7777a6e11613a8aa3a1e060a12af9f919342e7a8"/>
+             revision="e8e2fa3297aedd25081ccc3c269ad07d9d50a07e"/>
   </projects>
   <hooks>
     <hook name="install-environment"