Allow overriding of base URL from project
$CI_PAGES_URL will be set to the default Pages host, ignoring custom domains. This means that we get https://mesa.pages.freedesktop.org/mesa3d.org/ for the base URL when running in the main repository, which is not what we want.
Allow this to be overridden by a project setting to define the best canonical URL.
Signed-off-by: Daniel Stone daniels@collabora.com