Disable default, redundant NB.GV behavior

We already call `nbgv cloud` in the cloud builds, so there is no need to set certain pipeline variables with every project.
In fact given a couple bugs that have come into NB.GV lately around issues on [AzP](dotnet/Nerdbank.GitVersioning#675) or [GitHub Actions](dotnet/Nerdbank.GitVersioning#683) around setting pipeline variables, this should improve build reliability.

version.json

@@ -4,5 +4,8 @@
   "publicReleaseRefSpec": [
     "^refs/heads/main$",
     "^refs/heads/v\\d+(?:\\.\\d+)?$"
-  ]
+  ],
+  "cloudBuild": {
+    "setVersionVariables": false
+  }
 }