[roll] Roll Rust toolchain

Test: Ran 'fx clean-build x64' locally
Change-Id: I3671c1d420ef96e5fd79c19076491f7fd51e895c
diff --git a/fuchsia.ensure b/fuchsia.ensure
index 75537c2..a75a824 100644
--- a/fuchsia.ensure
+++ b/fuchsia.ensure
@@ -43,7 +43,7 @@
 
 # Rust
 @Subdir rust
-fuchsia/rust/${platform} git_revision:862703e05e275d77b0b594bb5d1a26a6340933f2
+fuchsia/rust/${platform} git_revision:8acec1f9d0b40dde142e6c26d7358b9ab232d2b4
 
 # Swift
 @Subdir swift