[build] fix conflict between bulid and jars tasks

The file `backend.native/build/libs/backend.native.jar` was produced
by two different tasks `:backend.native:jar` (default task of Java
Plugin) and `backend.native:jars` (our custom task). This caused the
`:dist: task to never be UP-TO-DATE.

build.gradle

@@ -143,7 +143,7 @@ task cross_dist(dependsOn: "crossDist")
 task list_dist(dependsOn: "listDist")
 
 task distCompiler(type: Copy) {
-    dependsOn ':backend.native:jars'
+    dependsOn ':backend.native:jar'
     dependsOn ':utilities:jar'
     dependsOn ':klib:jar'
     dependsOn ':shared:jar'