Avoid writing license file to root directory in Unity license script

Writing the Unity license file to `/root/.local/share/unity3d/Unity/Unity_lic.ulf` assumes the script is running as root, which may not be the case. This can cause permission issues and reduce portability.

This issue was identified in [PR #83](https://github.com/game-ci/unity-orb/pull/83) in [this comment](https://github.com/game-ci/unity-orb/pull/83#discussion_r1853245374) by @GabLeRoux.

We should consider updating the script to store the license file in the current user's home directory instead.