prometheus: use finalAttrs to make the version and webui overridable

This makes it possible to override the version and have the right webui
assets.
It also makes sure that the right version is used in other places of the
derivation.

Overriding now can be done like this:

```nix
final: prev:

  prometheus = prev.prometheus.overrideAttrs (
    finalAttrs: prevAttrs: {
      version = "3.1.0";
      src = final.fetchFromGitHub {
        owner = "prometheus";
        repo = "prometheus";
        tag = "v${finalAttrs.version}";
        hash = "";
      };
      vendorHash = "";

      webUiStatic = final.fetchurl {
        url = "https://github.com/prometheus/prometheus/releases/download/v${finalAttrs.version}/prometheus-web-ui-${finalAttrs.version}.tar.gz";
        hash = "";
      };
    }
  );
```

Whereas before you'd have to copy over things like the postPatch hook.

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