not ok - tbon.endpoint cannot be set

[grondo]

I've been seeing this failure regularly in CI, mainly in the inception builder for some reason:

2024-10-01T20:41:01.3261595Z flux-broker: setattr tbon.endpoint: File exists
2024-10-01T20:41:01.3262060Z 
2024-10-01T20:41:01.3262573Z flux-start: 0: PMI_Abort(): fatal bootstrap error
2024-10-01T20:41:01.3263033Z 
2024-10-01T20:41:01.3268384Z test_must_fail: died by non-SIGTERM signal: flux start -o,-Sbroker.rc1_path=,-Sbroker.rc3_path= -s2 -o,--setattr=tbon.endpoint=ipc:///tmp/customflux /bin/true
2024-10-01T20:41:01.3269615Z 
2024-10-01T20:41:01.3270381Z [1m[31mnot ok 18 - tbon.endpoint cannot be set(B[m
2024-10-01T20:41:01.3271468Z [36mnot ok 18 - tbon.endpoint cannot be set(B[m
2024-10-01T20:41:01.3272240Z #	
2024-10-01T20:41:01.3272847Z #		test_must_fail_or_be_terminated flux start ${ARGS} -s2 \
2024-10-01T20:41:01.3273804Z #			-o,--setattr=tbon.endpoint=ipc:///tmp/customflux /bin/true
2024-10-01T20:41:01.3275015Z #	
2024-10-01T20:41:01.3275224Z