Enable roll-forward so the tool works on .NET Core 3

Oren Novotny · Oren Novotny · commit ffc316a00789 · 2019-05-14T16:58:30.000-04:00
diff --git a/NuGetKeyVaultSignTool/runtimeconfig.template.json b/NuGetKeyVaultSignTool/runtimeconfig.template.json
@@ -0,0 +1,3 @@
+{
+  "rollForward": "Major"
+}
