Set PACT_TARGET_BRANCH correctly.

When the job runs on master, we need to publish the Pact as "master"
rather than "branch-master", so that the broker orders it correctly and
the publishing-api tests run against the correct branch.

Jenkinsfile

@@ -18,7 +18,8 @@ node {
     govuk.initializeParameters([
       'PUBLISHING_API_BRANCH': 'master',
     ])
-    govuk.setEnvar("PACT_TARGET_BRANCH", "branch-${env.BRANCH_NAME}")
+    def pact_branch = (env.BRANCH_NAME == 'master' ? 'master' : "branch-${env.BRANCH_NAME}")
+    govuk.setEnvar("PACT_TARGET_BRANCH", pact_branch)
     govuk.setEnvar("PACT_BROKER_BASE_URL", "https://pact-broker.dev.publishing.service.gov.uk")
 
     stage("Checkout gds-api-adapters") {