update: allow --nosign to work with only repo_pubkey set
update: allow --nosign to work with only repo_pubkey set

repo_pubkey is required for `fdroid update --nosign`, but repo_keyalias is
not.  For regular signing, the opposite true.

f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,f-droid/fdroid-server,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroid-server,fdroidtravis/fdroidserver,f-droid/fdroid-server,f-droid/fdroidserver