Changing to use File.OpenRead. This is required because when access readonly files, you must pass FileAccess.Read, otherwise, we get a UnauthorizedFileAccess exception.

src/Microsoft.DotNet.Cli.Utils/RuntimeConfig.cs

@@ -15,7 +15,7 @@ namespace Microsoft.DotNet.Cli.Utils
         public RuntimeConfig(string runtimeConfigPath) 
         {
             JObject runtimeConfigJson;
-            using (var streamReader = new StreamReader(new FileStream(runtimeConfigPath, FileMode.Open)))
+            using (var streamReader = new StreamReader(File.OpenRead(runtimeConfigPath)))
             {
                 runtimeConfigJson = OpenRuntimeConfig(streamReader);
             }