parent-uri attribute set with flux run flux start --test-size=

[grondo]

The detection of whether the current instance has a parent in order to set the parent-uri attribute currently relies on whether FLUX_JOB_ID is set in the environment. This doesn't always work because FLUX_JOB_ID can leak from the environment as is the case for flux run flux start -s 4 for example. (This is exercised by the inception tests)

Detection of instance as a job should instead rely on the jobid attribute, which is only set by a PMI boot.

[grondo]

Somehow github opened a duplicate #6560 and I already referenced that one in a PR so closing this.