chromium: replace update.nix with Python impl

update.nix was a huuuuge hack, abusing checksum collisions, etc., and
was extremely difficult to read and maintain, especially because
values from update.nix were also used in the derivations themselves!

I've replaced this with an implementation in Python, which I chose for
readability.  Rather than generating Nix, I chose to
generate JSON, since Python can do that in the standard library and
Nix can read it.

I also set update.py as an updateScript, so Chromium can now
automatically be updated!

Fixes: https://github.com/NixOS/nixpkgs/issues/89635

pkgs/applications/networking/browsers/chromium/default.nix

@@ -1,5 +1,5 @@
-{ newScope, config, stdenv, llvmPackages_10, llvmPackages_11
-, makeWrapper, ed, gnugrep, coreutils
+{ newScope, config, stdenv, fetchurl, makeWrapper
+, llvmPackages_10, llvmPackages_11, ed, gnugrep, coreutils
 , glib, gtk3, gnome3, gsettings-desktop-schemas, gn, fetchgit
 , libva ? null
 , pipewire_0_2
@@ -31,10 +31,11 @@ let
   chromium = rec {
     inherit stdenv llvmPackages;
 
-    upstream-info = (callPackage ./update.nix {}).getChannel channel;
+    upstream-info = (lib.importJSON ./upstream-info.json).${channel};
 
     mkChromiumDerivation = callPackage ./common.nix ({
-      inherit gnome gnomeSupport gnomeKeyringSupport proprietaryCodecs cupsSupport pulseSupport useOzone;
+      inherit channel gnome gnomeSupport gnomeKeyringSupport proprietaryCodecs
+              cupsSupport pulseSupport useOzone;
       # TODO: Remove after we can update gn for the stable channel (backward incompatible changes):
       gnChromium = gn.overrideAttrs (oldAttrs: {
         version = "2020-05-19";
@@ -63,22 +64,33 @@ let
     };
   };
 
+  pkgSuffix = if channel == "dev" then "unstable" else channel;
+  pkgName = "google-chrome-${pkgSuffix}";
+  chromeSrc = fetchurl {
+    url = map (repo: "${repo}/${pkgName}/${pkgName}_${version}-1_amd64.deb") [
+      "https://dl.google.com/linux/chrome/deb/pool/main/g"
+      "http://95.31.35.30/chrome/pool/main/g"
+      "http://mirror.pcbeta.com/google/chrome/deb/pool/main/g"
+      "http://repo.fdzh.org/chrome/deb/pool/main/g"
+    ];
+    sha256 = chromium.upstream-info.sha256bin64;
+  };
+
   mkrpath = p: "${lib.makeSearchPathOutput "lib" "lib64" p}:${lib.makeLibraryPath p}";
-  widevineCdm = let upstream-info = chromium.upstream-info; in stdenv.mkDerivation {
+  widevineCdm = stdenv.mkDerivation {
     name = "chrome-widevine-cdm";
 
-    # The .deb file for Google Chrome
-    src = upstream-info.binary;
+    src = chromeSrc;
 
     phases = [ "unpackPhase" "patchPhase" "installPhase" "checkPhase" ];
 
     unpackCmd = let
       widevineCdmPath =
-        if upstream-info.channel == "stable" then
+        if channel == "stable" then
           "./opt/google/chrome/WidevineCdm"
-        else if upstream-info.channel == "beta" then
+        else if channel == "beta" then
           "./opt/google/chrome-beta/WidevineCdm"
-        else if upstream-info.channel == "dev" then
+        else if channel == "dev" then
           "./opt/google/chrome-unstable/WidevineCdm"
         else
           throw "Unknown chromium channel.";
@@ -211,6 +223,7 @@ in stdenv.mkDerivation {
   passthru = {
     inherit (chromium) upstream-info browser;
     mkDerivation = chromium.mkChromiumDerivation;
-    inherit sandboxExecutableName;
+    inherit chromeSrc sandboxExecutableName;
+    updateScript = ./update.py;
   };
 }