Scope Linux-only dependencies to Linux platforms

[Apprentice2907]

This PR fixes cross-platform installation failures on Windows and macOS caused by Linux-only dependencies listed in dependency files.

Although PR #97 removed the apturl blocker, installation still fails on non-Linux systems because the following Ubuntu/Linux-specific packages are included unconditionally:
Brlapi
python-apt
systemd-python
These packages are not available on PyPI for Windows or macOS, which causes pip install to fail.

What changed
Added platform_system == "Linux" markers to Linux-only dependencies in:
requirements.txt
simulations.txt

This ensures:
Dependencies are installed normally on Linux
pip safely skips them on Windows/macOS

How to reproduce
On Windows/macOS:
pip install -r requirements.txt

Before: Installation fails with
No matching distribution found for Brlapi

After: Installation proceeds successfully
Environment
OS: Windows 10
Python: 3.11.9
Fresh virtual environment
Related issue

Fixes #103

[Apprentice2907]

Hi, I’ve opened a PR that scopes Linux-only dependencies (Brlapi, python-apt, systemd-python) using platform markers in both requirements.txt and simulations.txt.

This fixes the cross-platform installation failure on Windows/macOS while preserving Linux compatibility.

Please let me know if you’d like any changes. Thanks!