flux-environment(7): add SYSTEM SCRIPTS section #2501

Job	Run time
validate commits	0s
Generate build matrix	11s
python linting	30s
spelling	5s
bookworm - test-install	12m 35s
bookworm - 32 bit	13m 9s
noble - test-install	12m 49s
el9 - test-install	12m 15s
alpine - test-install	13m 1s
fedora40 - test-install	12m 3s
jammy - test-install	11m 40s
focal - py3.8	14m 28s
bookworm - gcc-12,content-s3,distcheck	14m 28s
fedora40 - clang-18	14m 28s
coverage	14m 29s
el8 - ascii	14m 29s
el8 - test-install	13m 9s
el8 - system,coverage	7m 35s
inception	14m 26s
flux-sched check	7m 11s
flux-pam check	4m 6s
flux-accounting check	4m 44s
flux-pmix check	10m 6s
macos build only	6m 9s
Generate docker manifest	1s
	3h 48m 7s

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