Show which CI job currently uses the resource group

Idea from Job gets stuck in "Waiting for Resource" (#381139 - closed)

Video demo: https://www.loom.com/share/488353a625114ef0aa76cea5f0ee8230

Currently, we only say which resource_group CI job is waiting for, but we don't show which CI job is actually using the resource_group right now. This makes it hard for clients to debug their CI pipelines, and also makes it hard for us to validate bugs like Job gets stuck in "Waiting for Resource" (#381139 - closed):

Suggestion

Add a reference to the job which is currently blocking the resource group.