qsyncthingtray: fix program paths

[peterhoeg]

Motivation for this change

We need this for qsyncthingtray to be able to detect the processes when the
binaries (syncthing, -inotify and pgrep) are not in the global path.

Any comments @zraexy ?

Things done

• Tested using sandboxing
  (nix.useSandbox on NixOS,
  or option build-use-sandbox in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • Linux
• 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.

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[mention-bot]

@peterhoeg, thanks for your PR! By analyzing the history of the files in this pull request, we identified @zraexy to be a potential reviewer.

[Mic92]

merge conflict?

[peterhoeg]

Thanks for noticing @Mic92.

[peterhoeg]

I didn't want to step on @zraexy's toes, which is why I didn't merge it straight away, but I guess if he has any issues with the changes, we can always address those. Thanks for your time on this PR, @Mic92 !

[zraexy]

@peterhoeg You don't have to worry about my toes. I've been pretty busy so I haven't had a chance to test the changes, but they look fine.