cerbero: Define a custom user agent while downloading
Some places block urllib, and others block wget / curl. For example, see: https://github.com/mesonbuild/libjpeg/issues/9
Set our own agent to workaround that, and so that people can contact us in case we're doing something they don't like while downloading things. I tested this by running fetch-package gstreamer-1.0
on an empty local_sources
directory which forced everything to be redownloaded.