Motivation for this change

The bspwm module doesn't start the process correctly. With the current behavior, you must kill both bspwm and sxhkd before the window manager fully quits.

Things done

Properly put the bspwm process in the background so that waitPID will be set correctly.

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

Additionaly, bspwm appears to have problems anyways when local config files are not present, but I haven't investigated this thoroughly enough.