ci: Increase workspace volume size

The default volume size created by the `dynamicPVC` function is not
sufficient to build the *metricspi* project.

ci/Jenkinsfile

@@ -5,7 +5,9 @@ pipeline {
         kubernetes {
             yamlFile 'ci/podTemplate.yaml'
             defaultContainer 'buildroot'
-            workspaceVolume dynamicPVC()
+            workspaceVolume dynamicPVC(
+                requestsSize: '20Gi',
+            )
         }
     }