adjustment and workaround for macOS iconv #221

Job	Run time
ci (tpb, ubuntu-22.04, --pb --threads --enable-libffi, test-some-fast)	28m 52s
ci (a6osx, macos-12)	17m 22s
ci (ta6osx, macos-12)	20m 47s
ci (arm64osx, macos-14)	8m 31s
ci (tarm64osx, macos-14)	10m 17s
ci (i3le, ubuntu-22.04)	11m 43s
ci (ti3le, ubuntu-22.04)	15m 8s
ci (a6le, ubuntu-22.04)	9m 1s
ci (ta6le, ubuntu-22.04, CFLAGS+=-std=gnu89)	11m 46s
ci (ta6le, ubuntu-22.04, reboot, -m=ta6le --force, re.boot SCHEME="chezscheme", test-some-fast)	7m 58s
ci (i3nt, windows-2022, vs)	11m 38s
ci (ti3nt, windows-2022, vs)	13m 58s
ci (a6nt, windows-2022, vs)	11m 42s
ci (ta6nt, windows-2022, vs)	15m 8s
ci (ta6nt, windows-2022, gcc, CFLAGS+=-std=gnu89)	13m 27s
	3h 27m 18s

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