Fix DOTNET_ROOT setting for dotnet run

Author: jjonescz

src/Cli/dotnet/Commands/Run/RunCommand.cs

@@ -454,7 +454,7 @@ static void SetRootVariableName(ICommand command, string runtimeIdentifier, stri
                 runtimeIdentifier,
                 defaultAppHostRuntimeIdentifier,
                 targetFrameworkVersion);
-            if (rootVariableName != null && Environment.GetEnvironmentVariable(rootVariableName) == null)
+            if (rootVariableName != null && string.IsNullOrEmpty(Environment.GetEnvironmentVariable(rootVariableName)))
             {
                 command.EnvironmentVariable(rootVariableName, Path.GetDirectoryName(new Muxer().MuxerPath));
             }

test/dotnet.Tests/CommandTests/Run/RunFileTests.cs

@@ -3044,6 +3044,10 @@ public void Api_RunCommand()
         string artifactsPath = OperatingSystem.IsWindows() ? @"C:\artifacts" : "/artifacts";
         string executablePath = OperatingSystem.IsWindows() ? @"C:\artifacts\bin\debug\Program.exe" : "/artifacts/bin/debug/Program";
         new DotnetCommand(Log, "run-api")
+            // The command outputs only _custom_ environment variables (not inherited ones),
+            // so make sure we don't pass DOTNET_ROOT_* so we can assert that it is set by the run command.
+            .WithEnvironmentVariable("DOTNET_ROOT", string.Empty)
+            .WithEnvironmentVariable($"DOTNET_ROOT_{RuntimeInformation.OSArchitecture.ToString().ToUpperInvariant()}", string.Empty)
             .WithStandardInput($$"""
                 {"$type":"GetRunCommand","EntryPointFileFullPath":{{ToJson(programPath)}},"ArtifactsPath":{{ToJson(artifactsPath)}}}
                 """)