Configuration cache failure when consuming root buildscript `classpath` configuration

When testing dependency guard on CI with configuration cache enabled, it seems to fail on a captured gradle script object. In this case, it's specifically when testing the buildscript classpath itself (literally the `classpath` configuration on the root project).

```
Task `:dependencyGuard` of type `com.dropbox.gradle.plugins.dependencyguard.internal.list.DependencyGuardListTask`: cannot serialize Gradle script object references as these are not supported with the configuration cache.
```

This is with 0.5.0