mastodon: refactor passthru on source

Any attribute concatenation onto the result of mkDerivation will break
*some* overrideability.

This case is rather obscure. You'd have to do something like this to get
an eval failure:

```
(mastodon.override { srcOverride = mastodon.src.overrideAttrs { }; })
```

This is not what I'am actually doing: I am calling `overrideAttrs` on
every `callPackage` call for an experiment - and this break with it.

pkgs/servers/mastodon/source.nix

@@ -7,17 +7,18 @@
 let
   version = "4.3.8";
 in
-(applyPatches {
+applyPatches {
   src = fetchFromGitHub {
     owner = "mastodon";
     repo = "mastodon";
     rev = "v${version}";
     hash = "sha256-08AApylDOz8oExZ0cRaZTgNAuP+1wiLkx0SDhkO2fMM=";
+
+    passthru = {
+      inherit version;
+      yarnHash = "sha256-IC4d/skIHEzJPuKlq4rMAqV+ydqquA6toq4WWCfuDxo=";
+      yarnMissingHashes = null;
+    };
   };
   patches = patches ++ [ ];
-})
-// {
-  inherit version;
-  yarnHash = "sha256-IC4d/skIHEzJPuKlq4rMAqV+ydqquA6toq4WWCfuDxo=";
-  yarnMissingHashes = null;
 }