Bump NVika tool to 4.0.0

Code quality CI runs have suddenly started failing out of nowhere:

- Passing run: https://github.com/ppy/osu/actions/runs/12806242929/job/35704267944#step:10:1
- Failing run: https://github.com/ppy/osu/actions/runs/12807108792/job/35707131634#step:10:1

In classic github fashion, they began rolling out another runner change
wherein `ubuntu-latest` has started meaning `ubuntu-24.04` rather than
`ubuntu-22.04`. `ubuntu-24.04` no longer has .NET 6 bundled.

Therefore, upgrade NVika to 4.0.0 because that version is compatible
with .NET 8.

.config/dotnet-tools.json

@@ -9,7 +9,7 @@
       ]
     },
     "nvika": {
-      "version": "3.0.0",
+      "version": "4.0.0",
       "commands": [
         "nvika"
       ]