fix: data race / nil channel read in pattern aggregation push

[na--]

What this PR does / why we need it:

This fixes a few different issues:

1. Setting p.quit = nil like that is actually a data race and the tests failed when executed with the -race flag:

   WARNING: DATA RACE
   Read at 0x00c000838508 by goroutine 82:
   github.com/grafana/loki/v3/pkg/pattern/aggregation.(*Push).run()
       github.com/grafana/loki/v3/pkg/pattern/aggregation/push.go:257 +0x184
   github.com/grafana/loki/v3/pkg/pattern/aggregation.Test_Push.func3.gowrap2()
       github.com/grafana/loki/v3/pkg/pattern/aggregation/push_test.go:172 +0x38
   
   Previous write at 0x00c000838508 by goroutine 80:
   github.com/grafana/loki/v3/pkg/pattern/aggregation.(*Push).Stop()
       github.com/grafana/loki/v3/pkg/pattern/aggregation/push.go:165 +0x1884
   github.com/grafana/loki/v3/pkg/pattern/aggregation.Test_Push.func3()
       github.com/grafana/loki/v3/pkg/pattern/aggregation/push_test.go:176 +0x1833
   testing.tRunner()
       testing/testing.go:1690 +0x226
   testing.(*T).Run.gowrap1()
       testing/testing.go:1743 +0x44
   

   This would be a problem not just in the tests, but in the actual execution of this module.
2. However, even if this had succeeded and the channel had been set to nil, that is even worse! Trying to read from a nil channel blocks forever, so the run() method would have blocked on case <-p.quit: instead of finished gracefully 😬
3. Finally, even if we somehow got to the desired result of Push.run() gracefully stopping, the Push.Stop() method didn't actually wait for that to happen, which might be breaking the expectation of upstream users of that method. It's generally a good idea to expect that whatever you have called the Stop() method of something, that thing has actually stopped once the method returns. It's much easier to reason about its behavior that way.

[trevorwhitney]

I misunderstood in slack, that the test fails with the -race flag before this change, but doesn't fail afterwards. That's all the test I need! LGTM