PHLEGM v7 validator false-positive fix

The build-architecture validator previously searched Compiler-UI.ps1 for the substring:
    Join-Path $projectRoot $relative

That incorrectly matched the legitimate build-script locator variable $relativeScript and aborted an otherwise-correct build.
The validator now checks for the exact obsolete final-install fallback expression instead.
The actual final install path remains supplied by the external bootstrap/Compiler UI as an absolute path.
