build-unit-test-docker: Update fmt 8.1.1 -> 9.1.0

meta-openembedded has already landed 9.1.0 and this will shortly be
incorportated into our tree.

Change-Id: I7ed38452f3715a7d002e378a52a19dbb9e4f8492
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 739f74a..36ecb0f 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -107,7 +107,7 @@
         ],
     ),
     "fmtlib/fmt": PackageDef(
-        rev="8.1.1",
+        rev="9.1.0",
         build_type="cmake",
         config_flags=[
             "-DFMT_DOC=OFF",