Fixes #13943.

I suspect my usage of sync.Cond was either wrong, naïve, or there's some behaviour in the runtime which is not as predictable as I'm expecting. I've changed to using a channel, which is simpler and what I should've done from the start.

I was able to provoke the flake reliably by running the following command, and with the fix I am not able to reproduce.
I don't 100% understand what causes the flake, but I'm fairly sure it's to do with the magic of sync.Cond.

$ go test -race -count=1000 -failfast ./coderd/notifications/... -run '^TestPendingUpdatesMetric$'
?       github.com/coder/coder/v2/coderd/notifications/types    [no test files]
ok      github.com/coder/coder/v2/coderd/notifications  155.644s
ok      github.com/coder/coder/v2/coderd/notifications/dispatch 2.585s [no tests to run]
ok      github.com/coder/coder/v2/coderd/notifications/render   1.647s [no tests to run]