format's V prefix parameter #290

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	30m 25s
ci (a6osx, macos-12)	16m 23s
ci (ta6osx, macos-12)	18m 27s
ci (arm64osx, macos-14)	10m 54s
ci (tarm64osx, macos-14)	10m 15s
ci (i3le, ubuntu-22.04)	11m 29s
ci (ti3le, ubuntu-22.04)	14m 41s
ci (a6le, ubuntu-22.04)	9m 15s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	11m 52s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	8m 14s
ci (i3nt, windows-2022, vs)	11m 51s
ci (ti3nt, windows-2022, vs)	14m 59s
ci (a6nt, windows-2022, vs)	11m 42s
ci (ta6nt, windows-2022, vs)	14m 50s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 38s
	3h 28m 55s

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