Let agents continuously report their health (woodpecker-ci#3893)

The agent should continue to report the health status as long as it got not terminated.

extracted from woodpecker-ci#2951

Right now there exist the case where you have ''shadow agents" who pull tasks even if they don't report healthy...

cmd/agent/core/agent.go

@@ -209,7 +209,6 @@ func run(c *cli.Context, backends []types.Backend) error {
 			err := client.ReportHealth(ctx)
 			if err != nil {
 				log.Err(err).Msg("failed to report health")
-				return
 			}
 
 			<-time.After(time.Second * 10)