flameshot: fix under KDE wayland

[oxalica]

Motivation for this change

KWin relies on absolute path in Exec= to find a process's corresponding desktop file and check if it's allowed to take screenshot. So we need to set Exec= to the absolute path instead of just flameshot.

KWin detail is discussed in flameshot-org/flameshot#1380

For testers: you may need to install it to environment.systemPackages and re-login to make it work.

cc: @jansol @haizaar

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• 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 wip"
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 21.11 Release Notes (or backporting 21.05 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.

[jansol]

Screenshot shortcut now works. It still takes a couple of seconds to actually respond to the key press but at least cropping is pretty much instant, unlike with Spectacle. Having two monitors seems to confuse flameshot greatly but that seems to be a known upstream issue. (flameshot-org/flameshot#1386)

[github-actions]

Successfully created backport PR #151074 for release-21.11.