Fixes bug in checking if ROS workspace exists in `install_deps.py` (#557)

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

<!-- As you go through the list, delete the ones that are not
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

tools/install_deps.py

@@ -112,13 +112,13 @@ def install_rosdep_packages(paths: list[str], ros_distro: str = "humble"):
                 if "isaac_lab_settings" in ext_toml and "ros_ws" in ext_toml["isaac_lab_settings"]:
                     # resolve the path to the ROS workspace
                     ws_path = ext_toml["isaac_lab_settings"]["ros_ws"]
-                    if not os.path.abspath(ws_path):
+                    if not os.path.isabs(ws_path):
                         ws_path = os.path.join(path, ws_path)
                     # check if the workspace exists
                     if not os.path.exists(f"{ws_path}/src"):
                         raise FileNotFoundError(
                             "During the installation of 'rosdep' dependencies, unable to find a"
-                            f" valid ROS workspace at: {path}/{ws_path}."
+                            f" valid ROS workspace at: {ws_path}."
                         )
                     # install rosdep if not already installed
                     if not os.path.exists("/etc/ros/rosdep/sources.list.d/20-default.list"):