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.

build.sbt

@@ -368,8 +368,19 @@ lazy val protobufV3 = pekkoModule("protobuf-v3")
     Compile / packageBin / packagedArtifact := Scoped.mkTuple2(
       (Compile / packageBin / artifact).value,
       ReproducibleBuildsPlugin.postProcessJar(OsgiKeys.bundle.value)),
-    Compile / packageBin := ReproducibleBuildsPlugin
-      .postProcessJar((Compile / assembly).value), // package by running assembly
+    Compile / packageBin := Def.taskDyn {
+      val store = streams.value.cacheStoreFactory.make("shaded-output")
+      val uberJarLocation = (assembly / assemblyOutputPath).value
+      val tracker = Tracked.outputChanged(store) { (changed: Boolean, file: File) =>
+        if (changed) {
+          Def.task {
+            val uberJar = (Compile / assembly).value
+            ReproducibleBuildsPlugin.postProcessJar(uberJar)
+          }
+        } else Def.task { file }
+      }
+      tracker(() => uberJarLocation)
+    }.value,
     // Prevent cyclic task dependencies, see https://github.com/sbt/sbt-assembly/issues/365 by
     // redefining the fullClasspath with just what we need to avoid the cyclic task dependency
     assembly / fullClasspath := (Runtime / managedClasspath).value ++ (Compile / products).value.map(Attributed.blank),

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 := {