matrix: demote fedora40 to x86_64 only #196

Job	Run time
spelling	5s
python linting	35s
Generate build matrix	11s
validate commits	0s
bookworm - test-install	1m 34s
bookworm - 32 bit	1m 35s
noble - test-install	1m 44s
el9 - test-install	1m 46s
alpine - test-install	1m 9s
fedora40 - test-install	1m 45s
jammy - test-install	1m 24s
focal - py3.8	1m 19s
bookworm - gcc-12,content-s3,distcheck	1m 48s
fedora40 - clang-18	1m 39s
coverage	1m 41s
el8 - ascii	1m 32s
el8 - system,coverage	1m 42s
inception	1m 35s
flux-pam check	1m 31s
flux-pmix check	1m 25s
flux-accounting check	1m 28s
flux-sched check	1m 24s
Generate docker manifest	0s
	28m 52s

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