Fix ImageBuilder Test Images after Arcade toolset layout change

[lbussell]

The docker-tools-imagebuilder-official pipeline is failing in Test Images after the Arcade update in #2099.

Failure from build 2972132: https://dev.azure.com/dnceng/internal/_build/results?buildId=2972132

src/run-tests.ps1 still reads the legacy Arcade toolset marker file directly:

docker-tools/src/run-tests.ps1

Lines 35 to 42 in ed4892e

	# Install additional runtimes from global.json. InitializeToolset resolves the Arcade
	# SDK but doesn't trigger restore. Build.proj delegates to Tools.proj for restore, which
	# imports InstallDotNetCore.targets to install additional runtimes from global.json.
	$globalJson = Get-Content (Join-Path $repoRoot 'global.json') | ConvertFrom-Json
	$arcadeSdkVersion = $globalJson.'msbuild-sdks'.'Microsoft.DotNet.Arcade.Sdk'
	# Need to use nested Join-Path calls to support Windows PowerShell, which doesn't support multiple paths in a single Join-Path call
	$toolsetLocationFile = Join-Path (Join-Path (Join-Path $repoRoot 'artifacts') 'toolset') "$arcadeSdkVersion.txt"
	$buildProj = Get-Content $toolsetLocationFile -TotalCount 1

Get-Content: /repo/src/run-tests.ps1:42
Cannot find path '/repo/artifacts/toolset/11.0.0-beta.26230.2.txt' because it does not exist.

#2099 updated Arcade to 11.0.0-beta.26230.2, including a toolset restore layout change that no longer writes artifacts/toolset/<version>.txt. This means pre-build validation fails before ImageBuilder tests run.

We should update the ImageBuilder test script to resolve the Arcade toolset through the current restore layout instead of assuming the legacy .txt marker file exists.