transmission: fix error when watch-dir is enabled

[HexF]

Motivation for this change

I discovered a bug causing an error when enabling watch-dir in the settings.

Failed to set up mount namespacing: /run/systemd/unit-root/var/lib/transmission/watchdir: No such file or directory

This patch simply creates the required folder into the activation script, similar to the incomplete-dir setting.

I'm not entirely sure if there is a relevant issue, but if there is please link it.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandbox = true set in nix.conf? (See Nix manual)
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution 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.

[ju1m]

@HexF, I'm sorry you had to face and fix this bug already fixed in #96655 a PR that I've been trying to get merged since a year.
@SuperSandro2000, if you feel like merging #96655, please do, otherwise merge this simpler one and I'll adapt the other.