tmsu: install manpage properly into man1 subdirectory

[KAction]

Motivation for this change

Without this change and MANPATH="$HOME/.nix-profile/share/man:", man
does not find tmsu manpage, which is installed as share/man/tmsu.1, not
share/man/man1/tmsu.1.

Things done

• [+] Tested using sandboxing (nix.useSandbox on NixOS, or option 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/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• [+] Assured whether relevant documentation is up to date
• [+] Fits CONTRIBUTING.md.

---