nixos: fix version checker (#1582)

Nixpkgs patches many libraries to read certificate bundle mentioned in
NIX_SSL_CERT_FILE, and /nix/store is already present in the sandbox.

Related to https://github.com/NixOS/nixpkgs/issues/409848

contrib/nixos/modules/copyparty.nix

@@ -285,6 +285,7 @@ in
 
         environment = {
           PYTHONUNBUFFERED = "true";
+          NIX_SSL_CERT_FILE = "${pkgs.cacert}/etc/ssl/certs/ca-bundle.crt";
           XDG_CONFIG_HOME = externalStateDir;
         };