Cache output of assembly

Previously, the build would rebuild over and over the uber jar with the
shaded protobuf3 distribution. With this patch, this is no longer the
case, as the output is cached locally.

project/Protobuf.scala

@@ -38,7 +38,7 @@ object Protobuf {
     outputPaths := Seq((Compile / sourceDirectory).value, (Test / sourceDirectory).value).map(_ / "java"),
     importPath := None,
     // this keeps intellij happy for files that use the shaded protobuf
-    Compile / unmanagedJars += (LocalProject("protobuf-v3") / assembly).value,
+    Compile / unmanagedJars += (LocalProject("protobuf-v3") / Compile / packageBin).value,
     protoc := "protoc",
     protocVersion := "3.11.4",
     generate := {