I'm adapting to the yarn changes too, (independent of the AUR) and uslug gives me some errors as well. For me, namely:

Error: uslug@https://github.com/laurent22/uslug.git#commit=8c12bc7678eaefa752e673ea9cfbc0b1a14d7237: EINVAL: invalid argument, rename '/tmp/xfs-5aa2219e' -> '/tmp/xfs-5aa2219e/package.tgz'

There's someone with the same issue on another package here; I'll try to find a resolution and maybe by lucky co-incidence it might help OP