fix PXR_PLUGINPATH_NAME and path appending

[antirotor]

This quick PR is fixing PXR_PLUGINPATH_NAME handling and path appending for PATH and PYTHONPATH

[Lypsolon]

I also would like to mention that this error appears at launcher startup.

Traceback (most recent call last):
  File "/home/lyonh/ynput/DevEnv/projects/AyonCore/ayon-core/client/ayon_core/addon/base.py", line 1446, in start_addons
    addon.tray_start()
  File "/home/lyonh/.local/share/AYON/addons/ayon_usd_1.0.2/ayon_usd/addon.py", line 37, in tray_start
    download_usd = is_usd_download_needed()
  File "/home/lyonh/.local/share/AYON/addons/ayon_usd_1.0.2/ayon_usd/utils.py", line 207, in is_usd_download_needed
    usd_root = get_downloaded_usd_root()
  File "/home/lyonh/.local/share/AYON/addons/ayon_usd_1.0.2/ayon_usd/utils.py", line 181, in get_downloaded_usd_root
    if existing_info["checksum"] != server_usd_info["checksum"]:
TypeError: 'NoneType' object is not subscribable

[Lypsolon]

tested under Almlinux9
the only thing I needed to "hack in" was a "solution" for setting LD_LIBRARY_PATH because it's erroring if the key doesn't already exist (will be fixed with an issue and PR)

noice work.

[Lypsolon]

Tested Works for AlmaLinux9
lets merge this and close the other PR.