[ssh] Wait for boot before attempting to SSH.

This rearranges the construction of the goroutines in execute() such
that we are no longer attempting to SSH before the system is paved and
has invoked mexec/reboot.

Change-Id: I2763ce687a55b60124853868e9243a710c1951f5
diff --git a/cmd/botanist/run.go b/cmd/botanist/run.go
index 82e6407..bb6cb46 100644
--- a/cmd/botanist/run.go
+++ b/cmd/botanist/run.go
@@ -283,13 +283,13 @@
 		if err := t.Start(ctx, imgs, r.zirconArgs); err != nil {
 			errs <- err
 		}
+		errs <- r.runCmd(ctx, args, t, syslog)
+	}()
+	go func() {
 		if err := t.Wait(ctx); err != nil && err != target.ErrUnimplemented {
 			errs <- err
 		}
 	}()
-	go func() {
-		errs <- r.runCmd(ctx, args, t, syslog)
-	}()
 
 	select {
 	case err := <-errs: