ci: Use a dynamic PVC for the workspace volume

Using an actual persistent volume for the workspace ultimately doesn't
work with multibranch pipelines.  When a branch/PR goes away and the
corresponding Jenkins job is deleted, the workspace cannot be cleaned up
because it is not mounted anywhere.  As such, we will just use a dynamic
PVC to ensure that the workspace is always cleaned up and nothing gets
left behind.  This of course obviates the need for the `Clean`
parameter.

ci/Jenkinsfile

@@ -5,9 +5,7 @@ pipeline {
         kubernetes {
             yamlFile 'ci/podTemplate.yaml'
             defaultContainer 'buildroot'
-            workspaceVolume persistentVolumeClaimWorkspaceVolume(
-                claimName: 'jenkins-ws-metricspi'
-            )
+            workspaceVolume dynamicPVC()
         }
     }
 
@@ -16,12 +14,6 @@ pipeline {
         disableConcurrentBuilds()
     }
 
-    parameters {
-        booleanParam \
-            name: 'Clean',
-            description: 'Clean the workspace and perform a full rebuild'
-    }
-
     environment {
         BUILDROOT_SRC = "${env.WORKSPACE}/buildroot"
         TMPDIR = "${env.WORKSPACE_TMP}"
@@ -32,11 +24,6 @@ pipeline {
     stages {
         stage('Prepare') {
             steps {
-                script {
-                    if (params.Clean) {
-                        sh 'rm -rf _build'
-                    }
-                }
                 checkout poll: false, scm: [
                     $class: 'GitSCM',
                     branches: [[name: '2022.05.x']],