parallel: fix calculation of max line length

parallel >= 20200822 uses /bin/echo to calculate the max allowed line
length. Patch it to a correct path, so that it doesn't (silently) fail
and fall back to a low value of 324.

Before:
  $ parallel --max-line-length-allowed
  324

After:
  $ parallel --max-line-length-allowed
  131063

Fixes: 16ca8725ff ("parallel: 20200722 -> 20200822")

pkgs/tools/misc/parallel/default.nix

@@ -1,4 +1,4 @@
-{ fetchurl, stdenv, perl, makeWrapper, procps }:
+{ fetchurl, stdenv, perl, makeWrapper, procps, coreutils }:
 
 stdenv.mkDerivation rec {
   name = "parallel-20200822";
@@ -8,6 +8,14 @@ stdenv.mkDerivation rec {
     sha256 = "02dy46g6f05p7s2qs8h6yg20p1zl3flxxf77n5jw74l3h1m24m4n";
   };
 
+  patches = [
+    ./fix-max-line-length-allowed.diff
+  ];
+
+  postPatch = ''
+    substituteInPlace src/parallel --subst-var-by coreutils ${coreutils}
+  '';
+
   outputs = [ "out" "man" ];
 
   nativeBuildInputs = [ makeWrapper ];