Use template method

[jasonvarga]

Disclaimer - I did this all in the GitHub editor. I didn't actually test it, so you'll probably want to check it out.

While I was doing https://github.com/statamic/cms/pull/5302 for Statamic 3.3, I went looking on GitHub for usages of augmentValue() and noticed this.

1. On Statamic 3.2, if you had a field in your blueprint named template, $this->augmentedValue('template') would have been a Value instance, so this condition would always be false.
2. On Statamic 3.3, even if you don't have a field named template, $this->augmentedValue('template') will be a Value instance, again making this condition always false.
3. You can just use the template() method to get the plain template name.
4. I swapped the instanceof check to first, so you can be sure the template() method will be available to use since you'll know it's a Page or Entry already.