Add env var to force use of system jsonnet libraries

Miniportile is used to retrieve jsonnet from github.
The progress reporter in miniportile can't handle a nil
content-length response which github sends with content-
encoding: Chunked.
There's a fix flavorjones/mini_portile#86
which needs merging into the gem at which point we can
remove this env var.

Jenkinsfile

@@ -64,6 +64,7 @@ node {
     }
 
     stage("bundle install") {
+      env.JSONNET_USE_SYSTEM_LIBRARIES = 1
       govuk.bundleApp();
     }