This package would sometimes require bison, depending on file enumeration order. Adding -a flag to cp to preserve mtimes fixes this.

Motivation for this change

Arcanist build was failing, bisecting nixpkgs turned up the first commit to cause a rebuild of it. It worked on some systems but not others. With this commit, it works on all systems again.

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)
• Fits CONTRIBUTING.md.