ci: build: verify downloaded toolchain tarball

ci: build: verify downloaded toolchain tarball

CDNs are known to ship outdated or corrupted files, if it unpacks
correctly, it necessarily doesn't mean, that we're using the desired
content. So lets fix it by checking the tarball as well.

I'm adding GPG checking explicitly, its not needed, but just double
checking, that everything is working as expected on build
infrastructure.

Signed-off-by: Petr Štetiar <ynezz@true.cz>