Cope with wrong modulepath

fixes issue reported by @besserox on the EasyBuild mailing list:

```

$ eb --debug -S GCC
== temporary log file in case of crash
/tmp/easybuild-oQ2Uot/easybuild-UpUFfC.log
ERROR: EasyBuild crashed with an error (at
easybuild/easybuild-develop/easybuild-framework/easybuild/tools/filetools.py:800
in mkdir): Failed to create directory
/usr/Modules/$MODULE_VERSION/modulefiles: [Errno 13] Permission
denied: '/usr/Modules'
```
yes, excellent remark, will fix