feat(image): expand ~ & $HOME in path

[zspher]

Hi, I've been trying to load a image with a relative path:

"image#logo": {
  "path": "~/.config/waybar/assets/nix-catppuccin-logo.svg"
},

but ~ and $HOME does not expand to the absolute path

so this PR adds the ff. changes

• adds ~ and $HOME expansion to image
• sets image only once
• minor change: exec has priority over path (if this needs to be changed please tell me)