Fix .desktop entry when installing in a UNIX system

[EwingKang]

Fix the path to the default Open3D application when installing to a custom path.

Type

• Bug fix (non-breaking change which fixes an issue):
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Motivation and Context

When installing to a custom path using cmake on the Ubuntu 22.04, desktop entry is not added correctly. The "Show Applications" button does not show the default Open3D viewer application.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

When building and installing to a custom path, the .desktop has incorrect path to the binary. It causes missing entry when using the application listing functions.
After this patch, it is successfully detected by the desktop system

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.