dotty phase 2: scalafix ExplicitNonNullaryApply (#28949)

* scalafix ExplicitNonNullaryApply prepare

+ Temporarily use com.sandinh:sbt-scalafix because scalacenter/scalafix#1098
+ Add ExplicitNonNullaryApply rule to .scalafix.conf
+ Manually fix a NonNullaryApply case in DeathWatchSpec that cause
  `fixall` fail because ExplicitNonNullaryApply rule incorrectly rewrite
  `context unbecome` to `context unbecome()` instead of `context.unbecome()`

* scalafix ExplicitNonNullaryApply

fix by enabling only ExplicitNonNullaryApply rule in .scalafix.conf then:
```
% sbt -Dakka.build.scalaVersion=2.13.1
> fixall
```

* scalafmtAll

* Revert to ch.epfl.scala:sbt-scalafix

Co-authored-by: Bùi Việt Thành <thanhbv@sandinh.net>

akka-cluster/src/multi-jvm/scala/akka/cluster/AttemptSysMsgRedeliverySpec.scala

@@ -51,7 +51,7 @@ class AttemptSysMsgRedeliverySpec
     }
 
     "redeliver system message after inactivity" taggedAs LongRunningTest in {
-      system.actorOf(Props[Echo], "echo")
+      system.actorOf(Props[Echo](), "echo")
       enterBarrier("echo-started")
 
       system.actorSelection(node(first) / "user" / "echo") ! Identify(None)