feat(burn-in): add fake build script for burnin-dummy repo; update default BuildCommand

gitea/burnin-dummy/build.ps1:
  Fake build script to commit to the burnin-dummy repo.
  Creates dist\build-info.txt (date, host, pid) after ~30s ping delay.
  Invoke-RemoteBuild packages dist\ as artifacts.zip.

Start-BurnInTest.ps1:
  Default BuildCommand updated from ping-only delay to build.ps1 invocation.
  Run with -NoSkipArtifact to exercise the full artifact pipeline.

scripts/Start-BurnInTest.ps1

@@ -48,7 +48,7 @@
 param(
     [int]    $Rounds              = 3,
     [int]    $Parallelism         = 4,
-    [string] $BuildCommand        = 'ping 127.0.0.1 -n 31 > nul',
+    [string] $BuildCommand        = 'powershell -NoProfile -NonInteractive -ExecutionPolicy Bypass -File build.ps1',
     [switch] $NoSkipArtifact,
     [int]    $RoundTimeoutMinutes = 60
 )
@@ -59,7 +59,7 @@ $ErrorActionPreference = 'Stop'
 $scriptDir = Split-Path -Parent $MyInvocation.MyCommand.Path
 
 $burnInParams = @{
-    RepoUrl               = 'http://10.10.20.11:3100/ci/burnin-dummy.git'
+    RepoUrl               = 'https://gitea.emulab.it/Simone/burnin-dummy.git'
     Branch                = 'main'
     TemplatePath          = 'F:\CI\Templates\WinBuild2025\WinBuild2025.vmx'
     SnapshotName          = 'BaseClean'