duply: fix ftp usage

for ftp, duply depends on ltfp on PATH, which is not possible if the
wrapper resets PATH.

pkgs/tools/backup/duply/default.nix

@@ -20,7 +20,7 @@ stdenv.mkDerivation rec {
     mkdir -p "$out/bin"
     mkdir -p "$out/share/man/man1"
     install -vD duply "$out/bin"
-    wrapProgram "$out/bin/duply" --set PATH \
+    wrapProgram "$out/bin/duply" --prefix PATH : \
         ${lib.makeBinPath [ coreutils python3 duplicity gawk gnupg bash gnugrep txt2man which ]}
     "$out/bin/duply" txt2man > "$out/share/man/man1/duply.1"
   '';