build: `apt-get purge sudo` after using it for sudo= build field
build: `apt-get purge sudo` after using it for sudo= build field

Once `sudo` has been used to execute the commands in sudo=, then it should
be removed from the build VM.  That prevents any other part of the build
from using sudo.  That means that all commands run with `sudo` must be
committed to fdroiddata.git, making them very visible.

closes #424

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