Use XDG_CONFIG_HOME when creating welcome stubs in profile.d/toolbox.sh

[CrimsonVoid]

Describe the bug
profile.d/toolbox.sh is hardcoded to look for welcome stubs in ~/.config which doesn't play nice if the user has set XDG_CONFIG_HOME to a different directory.

Steps how to reproduce the behaviour

1. Set XDG_CONFIG_HOME to a non-default path (eg ~/.local/config)
2. Create/enter a new toolbox
3. Find welcome stubs in ~/.config/toolbox

Expected behaviour
Welcome files should be created relative to $XDG_CONFIG_HOME/toolbox

Actual behaviour
Welcome files were created in ~/.config/toolbox

Screenshots
n/a

Output of toolbox --version (v0.0.90+)
toolbox version 0.3

Toolbx package info (rpm -q toolbox)
toolbox-0.3-1.fc43.x86_64

Output of podman version

Client:        Podman Engine
Version:       5.7.1
API Version:   5.7.1
Go Version:    go1.25.4 X:nodwarf5
Git Commit:    f845d14e941889ba4c071f35233d09b29d363c75
Built:         Tue Dec  9 18:00:00 2025
Build Origin:  Fedora Project
OS/Arch:       linux/amd64

Podman package info (rpm -q podman)
podman-5.7.1-1.fc43.x86_64

Info about your OS
Fedora Silverblue 43

Additional context
n/a