LiveFS.requestBuild disregards metadata_override for already-pending checks

It's reasonable to dispatch multiple concurrent builds of the same LiveFS with different metadata_override parameters; those constitute different builds. (Canonical's web team has a service in development which does this.) However, Launchpad doesn't compare LiveFSBuild.metadata_override when determining whether a build is already pending, so it incorrectly raises LiveFSBuildAlreadyPending if there is a pending build with other parameters equal but a different metadata_override.