btrfs-assistant: fix package

[khaneliman]

Description of changes

Resolves #319577

• Fixes nix-shell -p invoke
• Fixes calling the launcher on wayland being treated as CLI error due to WAYLAND_DISPLAY not being checked in the program at runtime.

Will just need to remove the substitutions after https://gitlab.com/btrfs-assistant/btrfs-assistant/-/merge_requests/81 is upstreamed and in next release.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.11 Release Notes (or backporting 23.11 and 24.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[eclairevoyant]

please have formatting in a separate commit, hard to tell what was changed in the first commit

[khaneliman]

please have formatting in a separate commit, hard to tell what was changed in the first commit

Sorry, yeah.. normally good about that but didn't this time. Updated commits to be more clear.