fmt: pickup v8.1.1

update fmt submodule so we can use fmt::join() without defining
formatter specialization for types which already have `operator<<()`.

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

diff --git a/src/fmt b/src/fmt
index 427b53405442872decb69d9735db0fa64106ebe2..b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9 160000 (submodule)
--- a/src/fmt
+++ b/src/fmt
@@ -1 +1 @@
-Subproject commit 427b53405442872decb69d9735db0fa64106ebe2
+Subproject commit b6f4ceaed0a0a24ccf575fab6c56dd50ccf6f1a9