buildserver: include disorderfs to avoid sudo: in metadata

Right now, the only thing that Briar needs in sudo: in order to be built
reproducibly is `apt-get install disorderfs`.  This includes it in the
standard image to make it easy to spread that technique to many builds.

@grote

buildserver/provision-apt-get-install

@@ -53,6 +53,7 @@ packages="
  ca-certificates-java
  cmake
  curl
+ disorderfs
  expect
  faketime
  flex