metropolis/pkg/cmd: fix race condition

cmd.Wait should not be accessed concurrently.

Change-Id: Icaaa1a22b1d23211dc1259af6cf97463228944ae
Reviewed-on: https://review.monogon.dev/c/monogon/+/1824
Tested-by: Jenkins CI
Reviewed-by: Lorenz Brun <lorenz@monogon.tech>
diff --git a/metropolis/pkg/cmd/run.go b/metropolis/pkg/cmd/run.go
index 7834c99..808a7c0 100644
--- a/metropolis/pkg/cmd/run.go
+++ b/metropolis/pkg/cmd/run.go
@@ -72,7 +72,7 @@
 		case line := <-lineC:
 			if pf(line) {
 				cmd.Process.Kill()
-				cmd.Wait()
+				<-complC
 				return true, nil
 			}
 		case err := <-complC: