post ls1 customs: fix

- full path for muonCustoms; required to be able to inline this customise function
- necessary for the production of MC samples EXO-Fall13-00199 ---> EXO-Fall13-00236, see https://hypernews.cern.ch/HyperNews/CMS/get/prep-ops/1174/1/1/1/1/1/1/1/1/1.html