KDE Plasma - Set environment variable session wide

Create a .sh file with your variables in ~/.config/plasma-workspace/env/. Every .sh file in there will be executed when Plasma starts.

Example:

~/.config/plasma-workspace/env/set-default-vulkan-driver.sh

#!/bin/bash
# Use the Radeon Vulkan implementation as default
export AMD_VULKAN_ICD=RADV