iwyu.yml: use `fedora:rawhide` to get the very latest packages / dropped some workarounds [skip ci] #511

Job	Run time
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	4m 37s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	7m 32s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	5m 58s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	1m 3s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	59s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	2m 2s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	56s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	54s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	1m 16s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	1m 0s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	1m 9s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	1m 0s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	1m 31s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	1m 2s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	1m 28s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libc++)	1m 6s
iwyu (macos-26, libc++, -Xiwyu --mapping_file=$(realpath ./macos.imp))	6m 56s
iwyu (ubuntu-22.04, fedora:rawhide, libstdc++)	1m 9s
	1h 23m 14s

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