WIP: content: flush before checkpointing #10040

Job	Run time
validate commits	10s
Generate build matrix	11s
python linting	30s
spelling	5s
bookworm - test-install	11m 59s
bookworm - 32 bit	13m 4s
noble - test-install	13m 12s
el9 - test-install	12m 18s
alpine - test-install	12m 24s
fedora40 - test-install	11m 44s
jammy - test-install	11m 49s
focal - py3.8	17m 13s
bookworm - gcc-12,content-s3,distcheck	18m 57s
fedora40 - clang-18	16m 52s
coverage	24m 10s
el8 - ascii	19m 22s
el8 - test-install	13m 29s
el8 - system,coverage	7m 16s
inception	21m 25s
flux-pam check	4m 13s
flux-sched check	6m 50s
macos build only	3m 9s
flux-accounting check	4m 38s
flux-pmix check	13m 18s
Generate docker manifest	0s
	4h 18m 18s

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