Avoid prereleases

Avoid prerelease versions of the .NET SDK if installed locally.

Author: martincostello

global.json

@@ -1,6 +1,7 @@
 {
   "sdk": {
     "rollForward": "latestFeature",
-    "version": "8.0.101"
+    "version": "8.0.101",
+    "allowPrerelease": false
   }
 }