seblu-push: Remove PKGBUILD sourcing
When PKGBUILD has pkgver function, sourcing it is not enough to get the package version (used to guess the package name to push). In that case, we need to dowload the sources and execute code to compute this version. This is heavy and has security implication beyond push fonction. We could have a partial PKGBUILD sourcing support by only working with the few which doesn't have pkgver function. But this would lead to difficult to diagnosis and the feature not worth it.