Don't allow using pkgrel in source URL
Currently, there is at least one package where the pkgrel is used to build the source URL. This is a problem as it means we cannot easily rebuild the package since if we bump the pkgrel the source URL becomes invalid (see #2674 for a user having problems with this). The one package I'm aware of which does this is linux-postmarketos-qcom-sm7150 (@jiaxyga, @ungeskriptet, @Adrianyyy) but it is possible there are others.
Ideally we would enforce this via CI, but I'm not sure how we could go about doing that in a reliable way given that APKBUILDs just are shell scripts.
One possible implementation could be to evaluate the value of the source
-field and check if any of the variables contain the substring "rX" (where X is the current value of the pkgrel), but I feel like this would be prone to false-positives. To avoid this, something more rigorous could be implemented like doing the previous step, and then incrementing the pkgrel and seeing if the condition still holds true. However, this would mean a lot of engineering effort invested into something that is a relatively minor problem which likely could be fixed on a case-by-case basis.