offload-build: retrieve the PKGBUILD in case makepkg changes it via pkgver
Building a package may change the PKGBUILD during update_pkgver. Let's retrieve the PKGBUILD after building to ensure we have the very same file as the one we used to build the package. Otherwise this may lead to the inability to distribute the package during commitpkg in case the expected and the actual hashsum mismatch.