Portal 2 mods no longer open on Ubuntu 20.04

[TheAwesome98-Real]

Your system information

• Steam client version (build number or date): Built May 27 2021, at 20:18:19
• Distribution (e.g. Ubuntu): Ubuntu 20.04.2 LTS (amd64)
• Opted into Steam client beta?: [Yes]
• Have you checked for system updates?: [Yes]

Please describe your issue in as much detail as possible:

The mod should open, however Portal 2 says "running" then stops immediately.

Steps for reproducing this issue:

1. Create a new Portal 2 mod following this tutorial.
2. Run the mod
3. The mod won't open

[MrSoup678]

I can confirm this. I tried to use Portal Unity Reboot on linux to this exact effect. Even more I tried the PortalGun HD mod, followed its instructions, and got my game muted.

Apparently implicit paths (without |gamepath_info| or |all_source_engine_paths|) are erroneously resolved in linux. They should fall back to |all_source_engine_paths|.