obmc-build-script: Update fmt to `8.1.1`

Match the version in
https://github.com/openbmc/openbmc/blob/595f63084d313c9708cb435470e6c6dcbbbbe646/meta-openembedded/meta-oe/recipes-support/fmt/fmt_8.1.1.bb

Change-Id: Ib647c4f54dfb1a6e619200247a6c4a9f95037140
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index db8004a..32ca42a 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -122,7 +122,7 @@
         ],
     ),
     "fmtlib/fmt": PackageDef(
-        rev="7.1.3",
+        rev="8.1.1",
         build_type="cmake",
         config_flags=[
             "-DFMT_DOC=OFF",