azure-pipeline.yml: add downloadBranch for release branch case

[AAndrisa]

PR Description

Add variable initialization for release branch case

PR Type

• [ ] Bug fix (a change that fixes an issue)
• [ ] New feature (a change that adds new functionality)
• [x] Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• [x] I have followed the coding standards and guidelines
• [x] I have conducted a self-review of my own code changes
• [ ] I have commented new code, particulary complex or unclear areas
• [ ] I have built libad9361-iio and check no new warnings/errors were introduced
• [ ] I have checked that my changes did not broke components that use libad9361-iio as dependency
• [ ] I have updated the documentation accordingly (GitHub Pages, READMEs, etc)