umount.davfs2 uses ps to get a process list to terminate gracefully.

On NixOS, this currently fails:

sh: ps: command not found
/run/current-system/sw/bin/umount.davfs:
  can't find mount.davfs-process with pid 4085;
  trying to unmount anyway.
  you propably have to remove /var/run/mount.davfs/root-x.pid manually
sh: umount: command not found

Fix this by patching ${procps}/bin/ps into the ps_command.

Afterwards:

umount.davfs: waiting for mount.davfs (pid 4106) to terminate gracefully .. OK

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.