Fix: central package version error (#41933)

<!-- Enter a brief description/summary of your PR here. What does it
fix/what does it change/how was it tested (even manually, if necessary)?
-->
## Summary of the Pull Request
This PR updates the `BuildXamlIndexBeforeSettings` target to avoid
running during DesignTimeBuild.
Previously, the target was triggered before `CoreCompile`, which caused
Visual Studio design-time builds to also invoke
`Settings.UI.XamlIndexBuilder`.
In design-time builds, the subproject may not fully inherit the central
package version management configuration (e.g.,
`Directory.Packages.props` not included, or incomplete MSBuild property
propagation).
As a result, NuGet central package version management did not fully
apply in design-time context, leading to false error such as:
<img width="1647" height="275" alt="image"
src="https://github.com/user-attachments/assets/24174c84-6de0-41be-ab94-8e853a66c5be"
/>

<!-- Please review the items on the PR checklist before submitting-->
## PR Checklist

- [ ] Closes: #xxx
- [ ] **Communication:** I've discussed this with core contributors
already. If the work hasn't been agreed, this work might be rejected
- [ ] **Tests:** Added/updated and all pass
- [ ] **Localization:** All end-user-facing strings can be localized
- [ ] **Dev docs:** Added/updated
- [ ] **New binaries:** Added on the required places
- [ ] [JSON for
signing](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ESRPSigning_core.json)
for new binaries
- [ ] [WXS for
installer](https://github.com/microsoft/PowerToys/blob/main/installer/PowerToysSetup/Product.wxs)
for new binaries and localization folder
- [ ] [YML for CI
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/ci/templates/build-powertoys-steps.yml)
for new test projects
- [ ] [YML for signed
pipeline](https://github.com/microsoft/PowerToys/blob/main/.pipelines/release.yml)
- [ ] **Documentation updated:** If checked, please file a pull request
on [our docs
repo](https://github.com/MicrosoftDocs/windows-uwp/tree/docs/hub/powertoys)
and link it here: #xxx

<!-- Provide a more detailed description of the PR, other things fixed,
or any additional comments/features here -->
## Detailed Description of the Pull Request / Additional comments

<!-- Describe how you validated the behavior. Add automated tests
wherever possible, but list manual validation steps taken as well -->
## Validation Steps Performed

src/settings-ui/Settings.UI/PowerToys.Settings.csproj

@@ -184,7 +184,7 @@
 		</Page>
 	</ItemGroup>
 
-	<Target Name="BuildXamlIndexBeforeSettings" BeforeTargets="CoreCompile">
+	<Target Name="BuildXamlIndexBeforeSettings" BeforeTargets="CoreCompile" Condition="'$(DesignTimeBuild)' != 'true'">
 		<Message Importance="high" Text="[Settings] Building XamlIndexBuilder prior to compile. Views='$(MSBuildProjectDirectory)\SettingsXAML\Views' Out='$(GeneratedJsonFile)'" />
 		<MSBuild Projects="..\Settings.UI.XamlIndexBuilder\Settings.UI.XamlIndexBuilder.csproj" Targets="Build" Properties="Configuration=$(Configuration);Platform=Any CPU;TargetFramework=net9.0;XamlViewsDir=$(MSBuildProjectDirectory)\SettingsXAML\Views;GeneratedJsonFile=$(GeneratedJsonFile)" />
 	</Target>