Run CommandMap with context class loader

[basil]

Fixes errors like this when using CommandMap with a context class loader that doesn't contain JavaMail classes:

DEBUG SMTP: IOException while sending, closing, THROW: 
javax.activation.UnsupportedDataTypeException: no object DCH for MIME type multipart/mixed; 
	boundary="----=_Part_5_726582541.1641663022150"
	at javax.activation.ObjectDataContentHandler.writeTo(DataHandler.java:909)
	at javax.activation.DataHandler.writeTo(DataHandler.java:330)
	at javax.mail.internet.MimeBodyPart.writeTo(MimeBodyPart.java:1694)
	at javax.mail.internet.MimeMessage.writeTo(MimeMessage.java:1913)
	at com.sun.mail.smtp.SMTPTransport.sendMessage(SMTPTransport.java:1315)
	at hudson.plugins.emailext.ExtendedEmailPublisher.sendMail(ExtendedEmailPublisher.java:524)
	at hudson.plugins.emailext.ExtendedEmailPublisher._perform(ExtendedEmailPublisher.java:444)
	at hudson.plugins.emailext.ExtendedEmailPublisher.perform(ExtendedEmailPublisher.java:354)
	at hudson.tasks.BuildStepMonitor$1.perform(BuildStepMonitor.java:20)
	at hudson.model.AbstractBuild$AbstractBuildExecution.perform(AbstractBuild.java:814)
	at hudson.model.AbstractBuild$AbstractBuildExecution.performAllBuildSteps(AbstractBuild.java:763)
	at hudson.model.Build$BuildExecution.cleanUp(Build.java:189)
	at hudson.model.Run.execute(Run.java:1943)
	at hudson.model.FreeStyleBuild.run(FreeStyleBuild.java:44)
	at hudson.model.ResourceController.execute(ResourceController.java:101)
	at hudson.model.Executor.run(Executor.java:442)