Ensure downloaded repositories GPG keys are valid

**Component**: packages

**What happened**:
We had an invalid GPG keys for SaltStack repository, because the URL has changed and we don't follow the redirection (`curl -L`).

**What was expected**:
The GPG key is valid and the installation is working.

**Steps to reproduce**:

**Resolution proposal** (optional):
Add a check at build time to ensure GPG keys are valid.
