ungoogled-chromium: Move ungoogled-src.nix into upstream-info.json

This also adds a dedicated channel for ungoogled-chromium that enables
us to update ungoogled-chromium independently of chromium.
TODO: Automate ungoogled-chromium updates via update.py (currently it
needs to be updated manually).

Note: Unfortunately this changes the ungoogled-chromium derivation
because common.nix passes the channel as an argument to
stdenv.mkDerivation (this makes it more difficult to verify this commit
but the result should remain the same).

pkgs/applications/networking/browsers/chromium/update.py

@@ -113,9 +113,12 @@ with open(JSON_PATH, 'w') as out:
             return 1
         elif channel_name == 'dev':
             return 2
+        elif channel_name == 'ungoogled-chromium':
+            return 3
         else:
             print(f'Error: Unexpected channel: {channel_name}', file=sys.stderr)
             sys.exit(1)
+    channels['ungoogled-chromium'] = last_channels['ungoogled-chromium'] # Keep ungoogled-chromium unchanged
     sorted_channels = OrderedDict(sorted(channels.items(), key=get_channel_key))
     json.dump(sorted_channels, out, indent=2)
     out.write('\n')