Update mujoco_env.py xml file reading

[KarahanS]

Description

Change: Avoid calling from_xml_path and instead first read the file and then call from_xml_string with the content.

Issue: Calling from_xml_path gives the following error (although the file exists):
ValueError: ParseXML: Error opening file 'C:\Users\karab\Desktop\Archive\Tübingen Archive\ML-4350 Reinforcement Learning\code\env\lib\site-packages\gymnasium\envs\mujoco\assets\half_cheetah.xml': No such file or directory

It may be because, MuJoCo's C++ backend might handle file paths differently than Python. (See google-deepmind/mujoco#257 (comment))

Python: 3.9.13
gymnasium: 1.0.0
mujoco: 3.2.6

Type of change

Please delete options that are not relevant.

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

[Kallinteris-Andreas]

are you using Microsoft's WSL?

[KarahanS]

I have WSL installed in my Windows machine, but I didn't run the code from WSL.

[Kallinteris-Andreas]

try (the original) with WSL
also what MuJoCo version are you using

[KarahanS]

I'm using mujoco 3.2.6.
I think requiring users to switch to WSL unnecessarily complicates the usage - that's why I opened the PR.

[Kallinteris-Andreas]

I just want to know if it's a Microsoft Windows issue Or the power shall issue.