[messaging] Fixed environment variables logic on Build Agent (#23676)

We were setting a custom home folder (.home directory defined in the
DOTNET_CUSTOM_HOME env var) when the dotnet SDK was installed by Pair To
Mac, to encapsulate caches and to not mix them with the global dotnet
installation:
https://github.com/dotnet/macios/blob/main/msbuild/Messaging/Xamarin.Messaging.Build/TaskRunner.cs#L62C4-L66C6

If the user assigns the global dotnet installation to the
"_DotNetRootRemoteDirectory" MSBuild property, the Build agent will end
up overriding the HOME folder with a custom home that doesn't exist
(there's no .home folder in the user profile in the Mac (where dotnet
installs the global caches).

This issue has been revealed from the recent changes that allows the
MSBuild client to bypass the "_DotNetRootRemoteDirectory" property to
the Build Agent:
https://dev.azure.com/devdiv/DevDiv/_git/ClientTools.Platform/pullrequest/663026

The fix consists of setting the custom home only if the dotnet SDK path
to use is not the global one

---------

Co-authored-by: GitHub Actions Autoformatter <[email protected]>

Author: mauroa

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}";