Configure DOTNET_ROOT when running apps (#16687)

The runtime uses DOTNET_ROOT to locate the .NET runtime. When unspecified,
the CLI will continue to use the runtime from the global default.

If you install dotnet to a location, configure the PATH, but not the runtime
then you end up running in to issues where it complains about missing runtime.

Author: pranavkm

src/BuiltInTools/dotnet-watch/HotReloadDotNetWatcher.cs

@@ -5,11 +5,14 @@
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Globalization;
+using System.IO;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.DotNet.Cli.Utils;
 using Microsoft.DotNet.Watcher.Internal;
 using Microsoft.DotNet.Watcher.Tools;
 using Microsoft.Extensions.Tools.Internal;
+using IReporter = Microsoft.Extensions.Tools.Internal.IReporter;
 
 namespace Microsoft.DotNet.Watcher
 {
@@ -82,7 +85,10 @@ public async Task WatchAsync(DotNetWatchContext context, CancellationToken cance
                     return;
                 }
 
-                ConfigureExecutable(context, processSpec);
+                if (context.Iteration == 0)
+                {
+                    ConfigureExecutable(context, processSpec);
+                }
 
                 using var currentRunCancellationSource = new CancellationTokenSource();
                 var forceReload = _console.ListenForForceReloadRequest();
@@ -200,6 +206,12 @@ private static void ConfigureExecutable(DotNetWatchContext context, ProcessSpec
                 processSpec.EnvironmentVariables["ASPNETCORE_URLS"] = context.DefaultLaunchSettingsProfile.ApplicationUrl;
             }
 
+            var rootVariableName = Environment.Is64BitProcess ? "DOTNET_ROOT" : "DOTNET_ROOT(x86)";
+            if (string.IsNullOrEmpty(Environment.GetEnvironmentVariable(rootVariableName)))
+            {
+                processSpec.EnvironmentVariables[rootVariableName] = Path.GetDirectoryName(new Muxer().MuxerPath);
+            }
+
             if (context.DefaultLaunchSettingsProfile.EnvironmentVariables is IDictionary<string, string> envVariables)
             {
                 foreach (var entry in context.DefaultLaunchSettingsProfile.EnvironmentVariables)