Comment 1 for bug 1702130

As I recall I got quite a way through debugging this and https://code.launchpad.net/~cjwatson/launchpad-buildd/local-snap-proxy/+merge/322545 was a big help, but the build logs in my local Launchpad instance suggest that that wasn't enough. With local-snap-proxy applied, it looks like I ended up with this:

maven-ant-tasks-download:
     [echo] Downloading Maven ANT Tasks...
      [get] Getting: http://repo2.maven.org/maven2/org/apache/maven/maven-ant-tasks/2.1.3/maven-ant-tasks-2.1.3.jar
      [get] To: /build/cassandra/parts/cassandra/build/build/maven-ant-tasks-2.1.3.jar
      [get] Error getting http://repo2.maven.org/maven2/org/apache/maven/maven-ant-tasks/2.1.3/maven-ant-tasks-2.1.3.jar to /build/cassandra/parts/cassandra/build/build/maven-ant-tasks-2.1.3.jar

BUILD FAILED
/build/cassandra/parts/cassandra/build/build.xml:277: java.net.ConnectException: Connection refused (Connection refused)
 at java.net.PlainSocketImpl.socketConnect(Native Method)
 at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
 at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
 at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
 at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
 at java.net.Socket.connect(Socket.java:589)
 at java.net.Socket.connect(Socket.java:538)
 at sun.net.NetworkClient.doConnect(NetworkClient.java:180)
 at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)
 at sun.net.www.http.HttpClient.openServer(HttpClient.java:527)
 at sun.net.www.http.HttpClient.<init>(HttpClient.java:211)
 at sun.net.www.http.HttpClient.New(HttpClient.java:308)
 at sun.net.www.http.HttpClient.New(HttpClient.java:326)
 at sun.net.www.protocol.http.HttpURLConnection.getNewHttpClient(HttpURLConnection.java:1202)
 at sun.net.www.protocol.http.HttpURLConnection.plainConnect0(HttpURLConnection.java:1138)
 at sun.net.www.protocol.http.HttpURLConnection.plainConnect(HttpURLConnection.java:1032)
 at sun.net.www.protocol.http.HttpURLConnection.connect(HttpURLConnection.java:966)
 at org.apache.tools.ant.taskdefs.Get$GetThread.openConnection(Get.java:728)
 at org.apache.tools.ant.taskdefs.Get$GetThread.get(Get.java:641)
 at org.apache.tools.ant.taskdefs.Get$GetThread.run(Get.java:631)

Total time: 6 seconds
Command '['/bin/sh', '/tmp/tmpqg9m6cnl', 'ant', '-autoproxy', 'artifacts', '-Ddist.dir=/build/cassandra/parts/cassandra/install']' returned non-zero exit status 1

The good news is that my fixes were enough to get your Jenkins snap working, although you need to apply https://github.com/evandandrea/jenkins-snap/pull/3.