ci: use parameters set by github-pull-request-builder

The parameters are set in the environment. It seems the best way to
check if the parameters are available, is with `params.<arg> == null`.

See-also: https://www.jenkins.io/doc/book/pipeline/getting-started/#global-variable-reference#
Signed-off-by: Niels de Vos <ndevos@redhat.com>

containerized-tests.groovy

@@ -30,7 +30,7 @@ node('cico-workspace') {
 
 	try {
 		stage('prepare bare-metal machine') {
-			if ("${ghprbPullId}".length() != 0) {
+			if (params.ghprbPullId != null) {
 				ref = "pull/${ghprbPullId}/head"
 			}
 			sh 'scp -o UserKnownHostsFile=/dev/null -o StrictHostKeyChecking=no ./prepare.sh root@${CICO_NODE}:'

containerized-tests.yaml

@@ -18,10 +18,6 @@
               - ci/centos
       script-path: containerized-tests.groovy
       lightweight-checkout: true
-    parameters:
-      - string:
-          name: ghprbPullId
-          default: ''
     triggers:
       - github-pull-request:
           status-context: ci/centos/containerized-tests