discord-{ptb,canary}: update to latest versions and fixup updateScript

This commit:

- Moves the update script into the dir, out of the maintainers dir. This makes
  it more discoverable in general. It can also be invoked from anywhere to write
  to default.nix
- Swaps it to use the standardized `passthru.updateScript`. This means that
  eventually bots like `nixpkgs-update` will be able to handle it.
- Runs the script to upgrade to the latest versions

See https://nixos.org/nixpkgs/manual/#ssec-stdenv-attributes

pkgs/applications/networking/instant-messengers/discord/base.nix

@@ -51,6 +51,8 @@ in stdenv.mkDerivation rec {
     mimeType = "x-scheme-handler/discord";
   };
 
+  passthru.updateScript = ./update-discord.sh;
+
   meta = with stdenv.lib; {
     description = "All-in-one cross-platform voice and text chat for gamers";
     homepage = "https://discordapp.com/";

pkgs/applications/networking/instant-messengers/discord/default.nix

@@ -1,5 +1,5 @@
 { branch ? "stable", pkgs }:
-# Generated by /maintainers/scripts/update-discord
+# Generated by ./update-discord.sh
 let
   inherit (pkgs) callPackage fetchurl;
 in {
@@ -17,20 +17,20 @@ in {
     pname = "discord-ptb";
     binaryName = "DiscordPTB";
     desktopName = "Discord PTB";
-    version = "0.0.18";
+    version = "0.0.19";
     src = fetchurl {
       url = "https://dl-ptb.discordapp.net/apps/linux/${version}/discord-ptb-${version}.tar.gz";
-      sha256 = "1v9xph7shpmabwysbc08qqlkydzgsir8rwarrjf01xmpcpyia17g";
+      sha256 = "0wixmm16qsmgbr0l9144vl5pc3s2p4jqhl7cnc6kmgy1y4ig56ib";
     };
   };
   canary = callPackage ./base.nix rec {
     pname = "discord-canary";
     binaryName = "DiscordCanary";
     desktopName = "Discord Canary";
-    version = "0.0.102";
+    version = "0.0.103";
     src = fetchurl {
       url = "https://dl-canary.discordapp.net/apps/linux/${version}/discord-canary-${version}.tar.gz";
-      sha256 = "1zl53vprkwbkfn3v0lddpp1swignqjxksi4qxl5zqj7482j372a4";
+      sha256 = "1d95q75ak4z6wkxlgcmkl7yk20gl7zf568b0xslz42hwx032fn4z";
     };
   };
 }.${branch}

pkgs/applications/networking/instant-messengers/discord/update-discord.sh

@@ -1,12 +1,12 @@
 #!/usr/bin/env bash
-# script to generate `pkgs/networking/instant-messengers/discord/default.nix`
+# script to generate ./default.nix
 
 set -e
-exec >${1:?usage: $0 <output-file>}
+exec >"${BASH_SOURCE%/*}/default.nix"
 
 cat <<EOF
 { branch ? "stable", pkgs }:
-# Generated by /maintainers/scripts/update-discord
+# Generated by ./update-discord.sh
 let
   inherit (pkgs) callPackage fetchurl;
 in {