[wasm] WBT: Fix breaking tests

`Wasm.Build.Tests.WasmTemplateTests.RunWithDifferentAppBundleLocations`
broke with the latest sdk:

`/root/helix/work/workitem/e/dotnet-latest/sdk/8.0.100-preview.7.23362.1/Roslyn/Microsoft.CSharp.Core.targets(80,5): error MSB6004: The specified task executable location "/root/helix/work/workitem/e/dotnet-latest/dotnet/dotnet" is invalid.`

This was because dotnet/roslyn#68918 changed
to expect `DOTNET_ROOT` to be a directory instead of the exe path.

src/mono/wasm/Wasm.Build.Tests/Common/RunCommand.cs

@@ -11,7 +11,7 @@ public class RunCommand : DotNetCommand
  public RunCommand(BuildEnvironment buildEnv, ITestOutputHelper _testOutput, string label="") : base(buildEnv, _testOutput, false, label)
  {
  WithEnvironmentVariables(buildEnv.EnvVars);
- WithEnvironmentVariable("DOTNET_ROOT", buildEnv.DotNet);
+ WithEnvironmentVariable("DOTNET_ROOT", Path.GetDirectoryName(buildEnv.DotNet)!);
  WithEnvironmentVariable("DOTNET_INSTALL_DIR", Path.GetDirectoryName(buildEnv.DotNet)!);
  WithEnvironmentVariable("DOTNET_MULTILEVEL_LOOKUP", "0");
  WithEnvironmentVariable("DOTNET_SKIP_FIRST_TIME_EXPERIENCE", "1");