job-exec: improve drain message for unkillable procs #10020

Job	Run time
python linting	35s
validate commits	13s
Generate build matrix	7s
spelling	6s
bookworm - test-install	12m 3s
bookworm - 32 bit	13m 55s
noble - test-install	12m 42s
el9 - test-install	12m 29s
alpine - test-install	12m 57s
fedora40 - test-install	12m 0s
jammy - test-install	12m 8s
focal - py3.8	16m 36s
bookworm - gcc-12,content-s3,distcheck	19m 43s
fedora40 - clang-18	16m 54s
coverage	24m 32s
el8 - ascii	18m 41s
el8 - test-install	14m 9s
el8 - system,coverage	7m 25s
inception	21m 29s
macos build only	3m 19s
flux-accounting check	4m 38s
flux-pam check	4m 22s
flux-pmix check	13m 9s
flux-sched check	6m 50s
Generate docker manifest	1s
	4h 21m 3s

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