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>
1 file changed