Here's from my build log after xcodebuild successfully build a workspace.

Because the plugin has some trouble after I migrated from a custom build early version of your plugin (before it was in the official repo), I deleted the build directory manually. After that I rerun the build process and stumbled upon the following error message. IMO a delete operation should never block a build, because of not existing file. What's the point here? Deleting a non existing file does not exist, which was the intention of the delete operation.

15:44:11 Cleaning up previously generate .ipa files
15:44:11 FATAL: /Users/Shared/Jenkins/Home/jobs/myProject/workspace/myProject/build/Release_Hockey-iphoneos does not exist.
15:44:11 /Users/Shared/Jenkins/Home/jobs/myProject/workspace/myProject/build/Release_Hockey-iphoneos does not exist.
15:44:11 at org.apache.tools.ant.types.AbstractFileSet.getDirectoryScanner(AbstractFileSet.java:474)
15:44:11 at hudson.FilePath.glob(FilePath.java:1240)
15:44:11 at hudson.FilePath.access$600(FilePath.java:164)
15:44:11 at hudson.FilePath$23.invoke(FilePath.java:1219)
15:44:11 at hudson.FilePath$23.invoke(FilePath.java:1217)
15:44:12 at hudson.FilePath.act(FilePath.java:758)
15:44:12 at hudson.FilePath.act(FilePath.java:740)
15:44:12 at hudson.FilePath.list(FilePath.java:1217)
15:44:12 at hudson.FilePath.list(FilePath.java:1203)
15:44:12 at au.com.rayh.XCodeBuilder.perform(XCodeBuilder.java:356)
15:44:12 at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:19)
15:44:12 at hudson.model.AbstractBuild$AbstractRunner.perform(AbstractBuild.java:693)
15:44:12 at hudson.model.Build$RunnerImpl.build(Build.java:178)
15:44:12 at hudson.model.Build$RunnerImpl.doRun(Build.java:139)
15:44:12 at hudson.model.AbstractBuild$AbstractRunner.run(AbstractBuild.java:459)
15:44:12 at hudson.model.Run.run(Run.java:1376)
15:44:12 at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:46)
15:44:12 at hudson.model.ResourceController.execute(ResourceController.java:88)
15:44:12 at hudson.model.Executor.run(Executor.java:230)

By the way, the IPA creation looks a bit messy to me. xcodebuild's CONFIGURATION_BUILD_DIR states the following for my project:

setenv CONFIGURATION_BUILD_DIR /Users/Shared/Jenkins/Home/jobs/myProject/workspace/DerivedData/myProject/Build/Products/Release_Hockey-iphoneos

but xcrun tries to find the *.app file in /Users/Shared/Jenkins/Home/jobs/myProject/workspace/myProject/build/Release_Hockey-iphoneos (yeah, I didn't configure a special build output directory, it's empty, so the default value is $xcode_proj_dir/build ?!?).