[release/9.0.1xx] Bring messaging and build agent fixes (#23746)

Author: dalexsoto

msbuild/Directory.Build.props

@@ -15,7 +15,7 @@
 			https://dev.azure.com/azure-public/vside/_artifacts/feed/xamarin-impl/NuGet/Xamarin.Messaging.Client/
 
 		-->
-		<MessagingVersion Condition="'$(MessagingVersion)' == ''">[18.0.136-g9e200fbc6d]</MessagingVersion>
+		<MessagingVersion Condition="'$(MessagingVersion)' == ''">[18.0.280-ga047b642b9]</MessagingVersion>
 		<HotRestartVersion>[17.14.133-g001ce2ac7a]</HotRestartVersion>
 	</PropertyGroup>
 	<Import Project="$(MSBuildThisFileDirectory)../Directory.Build.props" />

msbuild/Messaging/Xamarin.Messaging.Build/TaskRunner.cs

@@ -60,15 +60,19 @@ void SetDotNetVariables ()
 			var xmaDotNetPath = default (string);
 
 			if (IsValidDotNetInstallation (xmaDotNetRootPath)) {
-				//If the XMA dotnet is already installed, we use it and also declare a custom home for it (for NuGet restore and caches)
-				Environment.SetEnvironmentVariable ("DOTNET_CUSTOM_HOME", Path.Combine (xmaSdkRootPath, ".home"));
 				xmaDotNetPath = GetDotNetPath (xmaDotNetRootPath);
 			} else {
 				//In case the XMA dotnet has not been installed yet, we use the default dotnet installation
 				xmaDotNetPath = GetDefaultDotNetPath ();
 				xmaDotNetRootPath = Path.GetDirectoryName (xmaDotNetPath);
 			}
 
+			//We want to define a custom home for dotnet only if it's the Pair To Mac SDK installation
+			if (xmaDotNetRootPath.StartsWith (MessagingContext.BasePath, StringComparison.OrdinalIgnoreCase)) {
+				//The custom home is used for storing caches and not mix them with the global installation (NuGet, dotnet, etc.)
+				Environment.SetEnvironmentVariable ("DOTNET_CUSTOM_HOME", Path.Combine (xmaSdkRootPath, ".home"));
+			}
+
 			var pathContent = GetPathContent ();
 			//We add the XMA dotnet path first so it has priority over the default dotnet installation
 			var newPathContent = $"{xmaDotNetRootPath}:{pathContent}";