[ui] Random exec assignment chooses from only those with a matching taskGroup, if provided

[philrenaud]

Resolves #10962

Previously, when selecting Exec from a group page in the UI (or selecting a task group/task from the exec window's left sidebar), we go over a given job's allocations until we find one that is active and whose name matches the selected one.

This becomes a problem in situations where a user has multiple (differently-named) groups, whose tasks share a name between them.

This PR filters down to the group name in question.

[github-actions]

Ember Test Audit comparison

	main	a492da7	change
passes	1548	1548	0
failures	0	0	0
flaky	0	0	0
duration	11m 00s 454ms	10m 59s 858ms	-596ms

[lgfa29]

LGTM! Probably worth backporting how far as possible.