On jenkins 2.x, with groovy plugin 2.0, running SystemGroovyScript I managed to get to build variables, as below:
def build = this.getProperty('binding').getVariable('build')
def listener = this.getProperty('binding').getVariable('listener')
def env = build.getEnvironment(listener)
println env.MY_VARIABLE
If you are using goovy from file, simple System.getenv('MY_VARIABLE')
is sufficient