[roll] Roll fuchsia [sys] TaskGroup spawning is synchronous

An async mutex is only useful for holding across an await point [1].
Changing the internal mutex in TaskGroup to a sync::Mutex will simplify
task spawning and reduce overhead.

[1]: https://docs.rs/tokio/latest/tokio/sync/struct.Mutex.html#which-kind-of-mutex-should-you-use

Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/913014
Original-Revision: 6533f5e9ff2fd79fffd81cab1d81d1303109ef1f
GitOrigin-RevId: 7f8d36fffb5efbe0b50d66d0458f74beccfe9337
Change-Id: Iee4c3eb674d7bdf07faf538a47ce3fb7265f4806
diff --git a/stem b/stem
index 67be6ab..c2b6279 100644
--- a/stem
+++ b/stem
@@ -11,7 +11,7 @@
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-             revision="65fc975bfc2e7282b10c5c20f3f3db9e4cde0f36"/>
+             revision="6533f5e9ff2fd79fffd81cab1d81d1303109ef1f"/>
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