chromiumOSPackages: use more stable update source

It has been explained to me that cros-omahaproxy reports which
versions are available to users, while cros-updates-serving reports
the latest builds available for each channel.  The latter is probably
better for our use case anyway, and apparently, while both aren't
officially supported, is less likely to randomly break.

So let's use that instead, even if it is much more annoying to parse.

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