buildserver: ensure no custom apt sources

Remove all apt package sources configuration files before setting up the
standard set of sources (trixie, trixie-updates, trixie-backports,
trixie-security) to make sure that only those are present by default.

buildserver/provision-apt-get-install

@@ -38,6 +38,9 @@ if echo $debian_mirror | grep '^https' 2>&1 > /dev/null; then
     apt-get install ca-certificates
 fi
 
+# ensure that only the configured sources are active
+rm -f /etc/apt/sources.list /etc/apt/sources.list.d/*.*
+
 cat << EOF > /etc/apt/sources.list.d/debian.sources
 Types: deb
 URIs: ${debian_mirror}