To get around sandboxing of SCM stored Groovy scripts, I recommend to run the script as Groovy Command (instead of Groovy Script file):
import hudson.FilePath
final GROOVY_SCRIPT = "workspace/relative/path/to/the/checked/out/groovy/script.groovy"
evaluate(new FilePath(build.workspace, GROOVY_SCRIPT).read().text)
in such case, the groovy script is transferred from the workspace to the Jenkins Master where it can be executed as a system Groovy Script
. The sandboxing is suppressed as long as the Use Groovy Sandbox is not checked.