crosvm: switch back to old git repo URL
Despite having moved the main repo to crosvm/crosvm, release branches
are still only being created on chromiumos/platform/crosvm. So we
should have crosvm/crosvm as the homepage, but fetch from
chromiumos/platform/crosvm.
Link: https://github.com/NixOS/nixpkgs/pull/193746
