buildserver: make sure everything is upgraded

I found this in the logs:

The following packages have been kept back:
  linux-image-amd64

Not sure why apt is holding it back, probably some unclean base image.
Work around it for now to make sure we have the newest kernel.

buildserver/provision-apt-get-install

@@ -115,6 +115,10 @@ apt-get upgrade
 # again after upgrade in case of keyring changes
 apt-get update || apt-get update
 
+# make sure everything is upgraded
+apt-get upgrade --download-only
+apt-get upgrade
+
 packages="
  androguard/trixie-backports
  apksigner