samples: Fix dotnet sample

When trying to run the dotnet sample the following error happened:

```
FailFast:
Couldn't find a valid ICU package installed on the system. Set the
configuration flag System.Globalization.Invariant to true if you want to
run with no globalization support.
```

By adding this file, the configuration flag mentioned in the error
message is set to true, and the sample works again. [1]

[1] dotnet/core#2186 (comment)

samples/languages/dotnet/HelloWorld/runtimeconfig.template.json

@@ -0,0 +1,5 @@
+{
+    "configProperties": {
+        "System.Globalization.Invariant": true
+    }
+}