allow absolute path in prepend_paths

This patch is required to avoid issues like

```
EasyBuildError: "EasyBuild crashed with an error (at easybuild/tools/build_log.py:69 in caller_info): Sanity check failed: loading fake module failed: 'EasyBuild crashed with an error (at easybuild/tools/build_log.py:69 in caller_info): Absolute path /homedir/licenses/roguewave/license.lic passed to prepend_paths which only expects relative paths.'"
```