fix: unblock events watch on context cancel

Fixes #4653

This fixes an issue when `etcd` get stuck in `Pre` state so that even
`bootstrap` request doesn't kill it. This looks like a cluster which
fails to bootstrap itself and get stuck on reboot.

Even though `Watch()` aborts on cancel, channel receive will block
forever as no events are delivered.

Signed-off-by: Andrey Smirnov <andrey.smirnov@talos-systems.com>
(cherry picked from commit 58892cd)

internal/app/machined/pkg/system/services/etcd.go

@@ -267,7 +267,11 @@ func generatePKI(r runtime.Runtime) (err error) {
 	var event state.Event
 
 	for {
-		event = <-watchCh
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		case event = <-watchCh:
+		}
 
 		if event.Type == state.Created || event.Type == state.Updated {
 			break