Fix Get-LatestPackageVersion ignoring prerelease flag and configfile in dotnet args

[Copilot]

Get-LatestPackageVersion in Update-DotNetSdkVersions.ps1 computed $prereleaseFlag and $configSourceFlag but never used them — $dotnetArgs always included --prerelease unconditionally, omitted --exact-match, and never passed --configfile. This could silently update MSBuild SDKs to prerelease versions and search the wrong feeds.

Changes

• Removed unused $prereleaseFlag, $configSourceFlag, and $searchCmd variables
• Replaced $configSourceFlag (embedded CLI string) with $configFilePath (plain path)
• Rebuilt $dotnetArgs from the actual computed values:

$dotnetArgs = @('package', 'search', $PackageId, '--exact-match', '--format', 'json')

if ($IncludePrerelease) { $dotnetArgs += '--prerelease' }

if ($configFilePath) {
    $dotnetArgs += '--configfile'
    $dotnetArgs += $configFilePath
}

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