[msbuild] Use exact version of XVS' packages. (#20043)

There are testing versions of the XVS' packages in some feeds with version
99.**, but we don't want to reference those by accident, so make sure we
reference the exact version of the XVS NuGet packages.

This has caused bugs multiple times in the past, the reference below is just
the latest one.

Ref: https://devdiv.visualstudio.com/DevDiv/_workitems/edit/1950696/?view=edit

msbuild/Directory.Build.props

@@ -1,7 +1,18 @@
 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
 	<PropertyGroup>
-		<MessagingVersion>1.13.8</MessagingVersion>
-		<HotRestartVersion>1.1.7</HotRestartVersion>
+		<!--
+
+			Some feeds have testing versions of the Messaging and HotRestart
+			packages with versions 99.*. This means that we'll want the exact
+			version of the packages below, not the specified version or any
+			other later version, because if the requested version doesn't
+			exist, we'll get the testing version and things break in
+			interesting ways. So in order to get the exact version, they're
+			enclosed in brackets.
+
+		-->
+		<MessagingVersion>[1.13.8]</MessagingVersion>
+		<HotRestartVersion>[1.1.7]</HotRestartVersion>
 	</PropertyGroup>
 	<Import Project="$(MSBuildThisFileDirectory)../Directory.Build.props" />
 </Project>