content: require backing store for checkpoint #10039

Job	Run time
validate commits	8s
spelling	8s
Generate build matrix	6s
python linting	39s
bookworm - test-install	12m 22s
bookworm - 32 bit	12m 57s
noble - test-install	13m 30s
el9 - test-install	12m 11s
alpine - test-install	12m 49s
fedora40 - test-install	11m 46s
jammy - test-install	11m 44s
focal - py3.8	17m 27s
bookworm - gcc-12,content-s3,distcheck	19m 32s
fedora40 - clang-18	16m 35s
coverage	23m 48s
el8 - ascii	19m 32s
el8 - test-install	13m 28s
el8 - system,coverage	7m 23s
inception	21m 37s
flux-accounting check	4m 44s
flux-pmix check	13m 12s
flux-pam check	4m 14s
macos build only	3m 32s
flux-sched check	6m 48s
Generate docker manifest	0s
	4h 20m 12s

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