ImageBuilder update does not preserve executable bits in bundled infrastructure #2181

The infrastructure files packed into ImageBuilder lose their Unix file mode metadata. When update recreates them, files tracked as 100755 are written without execute bits.

Currently affected:

• skill-helpers/Get-BuildLog.ps1
• skill-helpers/Get-FailingPipelines.ps1
• skill-helpers/Show-BuildTimeline.ps1
• skill-helpers/Show-PullRequestComments.ps1

This is not impactful today because these scripts are invoked through pwsh rather than executed directly.

Related NuGet issue: NuGet/Home#13121

Suggested fix: Generate a list of executable files at build time, embed it in ImageBuilder, and restore their execute bits in the update command.