launchpad: fix upload path

As of recently, launchpad rejects uploads that don't specify
their ppa name. This didn't affect nightlies because the ppa
name is explicit (sflphone-nightly), but did break stable releases
because we rely on the implicit default PPA name, "ppa".

Refs #51465

Change-Id: I1ae4f7f5e1de21960240a4a50a8896a4208c5ee5