Publish placeholder log artifact from Prepare Installers jobs

[Copilot]

The WinGet and Homebrew jobs in the Prepare Installers stage use the shared 1ES job template with build log publishing enabled, but they don't run MSBuild and so never create $(Build.ArtifactStagingDirectory)/artifacts/log/$(_BuildConfig), producing log artifact publishing noise/failures.

Description

• New template eng/pipelines/templates/prepare-installer-log-artifact.yml: single pwsh step (cross-platform — runs on both the Windows WinGet job and macOS Homebrew job) that creates the standard log artifact directory and writes an installer-job.txt marker explaining that installer preparation jobs produce no MSBuild logs.
• Wired in as the first step (after checkout) of the WinGet and Homebrew jobs in both eng/pipelines/azure-pipelines.yml (official) and eng/pipelines/azure-pipelines-unofficial.yml (unofficial).

steps:
  - checkout: self
    fetchDepth: 1
  - template: /eng/pipelines/templates/prepare-installer-log-artifact.yml@self
  - task: DownloadPipelineArtifact@2
    ...

Checklist

• Is this feature complete?
  • Yes. Ready to ship.
  • No. Follow-up changes expected.
• Are you including unit tests for the changes and scenario tests if relevant?
  • Yes
  • No
• Did you add public API?
  • Yes
    • If yes, did you have an API Review for it?
      • Yes
      • No
    • Did you add <remarks /> and <code /> elements on your triple slash comments?
      • Yes
      • No
  • No
• Does the change make any security assumptions or guarantees?
  • Yes
    • If yes, have you done a threat model and had a security review?
      • Yes
      • No
  • No
• Does the change require an update in our Aspire docs?
  • Yes
    • Link to aspire.dev issue:
      • New issue
  • No

[radical]

Fixed by #16865.