Currently, osbuild downloads are identified as coming from `curl`. This
is unfortunate because some RPM mirrors block requests from curl. Let's
"fix" that by introducing our own user-agent. While this can certainly
be seen as "circumventing" a policy, I think that this change is
actually helpful: Now, the mirror maintainers can actually distinguish
osbuild requests from regular curl calls. If they want to block osbuild,
they certainly can, we have no power there, but at least this allows
more fine-grained filtering. Also, our new user-agent contains our
domain name, so if there's a problem, they can contact us.
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