buildserver: fix apksigner install
buildserver: fix apksigner install

Adding /bullseye-backports confuses things apparently.  Without it, apt
will look in bullseye-backports automatically, including for deps.  But it
will not look for deps in bullseye-backports if /bullseye-backports is used.

!1205

f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver,f-droid/fdroidserver