Fix signing task onlyIf restriction (amazon-ion#483)

The argument to `hasTask` must be provided as either an instance of
`Task` or a `String` representing the absolute task path.

The absolute task path for the project root publish task is `:publish`.

build.gradle.kts

@@ -166,7 +166,7 @@ tasks {
     }
 
     withType<Sign> {
-        setOnlyIf { isReleaseVersion && gradle.taskGraph.hasTask("publish") }
+        setOnlyIf { isReleaseVersion && gradle.taskGraph.hasTask(":publish") }
     }
 
     cyclonedxBom {