Invoke-RemoteBuild.ps1: when BuildCommand succeeds (exit 0) but GuestArtifactSource
(default: dist) does not exist, instead of silently skipping packaging, now creates a
placeholder 'build-ok.txt' in the artifact output dir and zips it. This ensures
Get-BuildArtifacts.ps1 always finds artifacts.zip regardless of whether the build
produced real output files. Correct for burn-in, smoke, and delay-only build commands.
Start-BurnInTest.ps1: revert default BuildCommand to 'ping 127.0.0.1 -n 31 > nul'
(~30s delay, no artifact output) now that the pipeline handles missing dist gracefully.
The previous 'if not exist dist mkdir dist' workaround is no longer needed.