Fixes bug introduced in https://github.com/isaac-sim/IsaacLab/pull/621

The error was using `os.path.abspath()` instead of `os.path.isabs()` to
check if a path exists. This would always return a value (as it actually
converts to an absolute path) so the if branch was never entered for
relative paths.

Fixes #556

## Type of change

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applicable. -->

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there