improve management of systemd stop timeout to avoid SIGKILL while making progress #10346

Job	Run time
Generate build matrix	6s
python linting	32s
validate commits	16s
spelling	8s
bookworm - test-install	12m 38s
bookworm - 32 bit	13m 56s
noble - test-install	12m 48s
el9 - test-install	12m 49s
alpine - test-install	13m 27s
fedora40 - test-install	12m 31s
jammy - test-install	12m 12s
focal - py3.8	17m 30s
bookworm - gcc-12,content-s3,distcheck	20m 29s
fedora40 - clang-18	16m 54s
coverage	24m 47s
el8 - ascii	20m 3s
el8 - test-install	13m 28s
el8 - system,coverage	7m 23s
inception	22m 13s
macos build only	2m 56s
flux-pam check	4m 28s
flux-pmix check	13m 1s
flux-accounting check	4m 41s
flux-sched check	6m 58s
Generate docker manifest	1s
	4h 26m 15s

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