Clear MSBuildExtensionsPath as well when building/publishing test apps

Otherwise this will point to the path in the executing host sdk which causes issues when running x86 tests.

Author: akoeplinger

src/Microsoft.Diagnostics.TestHelpers/DotNetBuildDebuggeeTestStep.cs

@@ -200,6 +200,7 @@ protected async Task Restore(string extraArgs, ITestOutputHelper output)
                 .WithEnvironmentVariable("DOTNET_CLI_DO_NOT_USE_MSBUILD_SERVER", "true")
                 .WithEnvironmentVariable("DOTNET_MSBUILD_SDK_RESOLVER_CLI_DIR", Path.GetDirectoryName(DotNetToolPath))
                 .WithEnvironmentVariable("DOTNET_INSTALL_DIR", Path.GetDirectoryName(DotNetToolPath))
+                .RemoveEnvironmentVariable("MSBuildExtensionsPath")
                 .RemoveEnvironmentVariable("MSBuildSDKsPath")
                 .WithWorkingDirectory(DebuggeeSolutionDirPath)
                 .WithLog(output)
@@ -247,6 +248,7 @@ protected async Task Build(string dotnetArgs, ITestOutputHelper output)
                 .WithEnvironmentVariable("DOTNET_CLI_DO_NOT_USE_MSBUILD_SERVER", "true")
                 .WithEnvironmentVariable("DOTNET_MSBUILD_SDK_RESOLVER_CLI_DIR", Path.GetDirectoryName(DotNetToolPath))
                 .WithEnvironmentVariable("DOTNET_INSTALL_DIR", Path.GetDirectoryName(DotNetToolPath))
+                .RemoveEnvironmentVariable("MSBuildExtensionsPath")
                 .RemoveEnvironmentVariable("MSBuildSDKsPath")
                 .WithWorkingDirectory(DebuggeeProjectDirPath)
                 .WithLog(output)