Add build test for NAG

What is the new feature

Test builds for NAG compiler

How is it implemented

Add new stage build to pipeline

Mandatory steps before review

• Gitlab CI passes (Hint: use make format for linting)
• New feature is covered by additional unit tests
• Mark the merge request as ready by removing Draft:

Mandatory steps before merge

• Reviewed by a maintainer
• Incorporate review suggestions

You are not supposed to merge this request by yourself, the maintainers of fortan-support take care of this action!