[PERF] Explicitly set dotnet root when building tools (#88801)

Explicitly set DOTNET_ROOT for building performance tools.

Author: LoopedBard3

eng/pipelines/coreclr/templates/run-scenarios-job.yml

@@ -153,6 +153,10 @@ jobs:
     - script: cp -r $(PerformanceDirectory)/scripts $(WorkItemDirectory)/scripts/ && cp -r $(PerformanceDirectory)/src/scenarios/shared $(WorkItemDirectory)/shared/ && cp -r $(PerformanceDirectory)/src/scenarios/staticdeps/ $(WorkItemDirectory)/staticdeps/
       displayName: Copy scenario support files (Linux/MAC)
       condition: and(succeeded(), ne(variables['Agent.Os'], 'Windows_NT'))
+    - powershell: |
+        Write-Host "##vso[task.setvariable variable=DOTNET_ROOT;]$(PayloadDirectory)/dotnet"
+        Write-Host "Set DOTNET_ROOT to $(PayloadDirectory)/dotnet"
+      displayName: Explicitly set DOTNET_ROOT
     # build Startup
     - script: $(PayloadDirectory)\dotnet\dotnet.exe publish -c Release -o $(WorkItemDirectory)\Startup -f net7.0 -r win-$(Architecture) $(PerformanceDirectory)\src\tools\ScenarioMeasurement\Startup\Startup.csproj -p:DisableTransitiveFrameworkReferenceDownloads=true
       displayName: Build Startup tool (Windows)