.github: Change concurrency group ids so they're unique.

.github: Change concurrency group ids so they're unique.

GitHub strikes again.  Apparently the github.ref context variable only
contains the PR number if the workflow is triggered by "pull_request" so
since we just changed the trigger to "pull_request_target" the variable
no longer contains the PR number and is therefore not unique and can't be
used as a concurrency group id.  We now use
`github.triggering_actor-github.head_ref`.