Rename GIT_BRANCH variable to SOURCE_BRANCH in DSL

- GIT_BRANCH is passed from Github for PR builds. This
  param was overiding the default value of 'master' in
  PR builds which causes PRs to fail to find the ref.

Related eclipse-openj9#2836 eclipse-openj9#4443 eclipse-openj9#4783
[skip ci]

Signed-off-by: Adam Brousseau <adam.brousseau88@gmail.com>

buildenv/jenkins/jobs/pipelines/Pipeline_Template.groovy

@@ -28,7 +28,7 @@ if (!binding.hasVariable('VENDOR_BRANCH_DEFAULT')) VENDOR_BRANCH_DEFAULT = ''
 if (!binding.hasVariable('VENDOR_CREDENTIALS_ID_DEFAULT')) VENDOR_CREDENTIALS_ID_DEFAULT = ''
 if (!binding.hasVariable('DISCARDER_NUM_BUILDS')) DISCARDER_NUM_BUILDS = '1'
 if (!binding.hasVariable('GIT_URI')) GIT_URI = 'https://github.com/eclipse/openj9.git'
-if (!binding.hasVariable('GIT_BRANCH')) GIT_BRANCH = 'refs/heads/master'
+if (!binding.hasVariable('SOURCE_BRANCH')) SOURCE_BRANCH = 'refs/heads/master'
 if (!binding.hasVariable('GIT_REFSPEC')) GIT_REFSPEC = ''
 if (!binding.hasVariable('LIGHTWEIGHT_CHECKOUT')) LIGHTWEIGHT_CHECKOUT = true
 
@@ -50,7 +50,7 @@ pipelineJob("$JOB_NAME") {
                         url(GIT_URI)
                         refspec(GIT_REFSPEC)
                     }
-                    branch("${GIT_BRANCH}")
+                    branch("${SOURCE_BRANCH}")
                     extensions {
                         cleanBeforeCheckout()
                     }