Add FMT_FORCE_FALLBACK_FILE to force the use of fallback_file #6377

Job	Run time
mingw (mingw64)	4m 13s
build (windows-2019, Win32, v142, 17, Debug)	1m 18s
mingw (ucrt64)	4m 49s
build (windows-2019, Win32, v142, 17, Release)	2m 38s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 8s
build (windows-2019, Win32, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 18s
build (windows-2019, x64, v140, 14, Debug)	1m 13s
build (windows-2019, x64, v140, 14, Release)	1m 55s
build (windows-2019, x64, v141, 17, Debug)	1m 24s
build (windows-2019, x64, v141, 17, Release)	2m 24s
build (windows-2019, x64, v142, 17, Debug)	1m 24s
build (windows-2019, x64, v142, 17, Release)	2m 22s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Debug)	1m 12s
build (windows-2019, x64, v142, 17, -DBUILD_SHARED_LIBS=ON, Release)	2m 40s
build (windows-2019, x64, v142, 20, Debug)	2m 9s
build (windows-2019, x64, v142, 20, Release)	2m 43s
build (windows-2022, x64, v143, Debug, 20)	1m 47s
	37m 37s

Provide feedback