Fix pipeline with waskd2 exp (#44334)

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## Summary of the Pull Request
This pull request updates the WinAppSDK version used in the build
pipeline to 1.8 and makes related improvements to the NuGet restore
process and configuration handling.

Version update:

* Updated the default value of the `winAppSdkVersionNumber` parameter
from `"1.7"` to `"1.8"` in `.pipelines/UpdateVersions.ps1`, ensuring the
pipeline uses the latest WinAppSDK version.

NuGet restore and configuration improvements:

* Changed the `Add-NuGetSourceAndMapping` call in the
`Resolve-WinAppSdkSplitDependencies` function to use the `$installDir`
variable instead of a hardcoded path, improving flexibility for local
package mapping in `.pipelines/UpdateVersions.ps1`.
* Added a `workingDirectory` property to the NuGet restore step in
`.pipelines/v2/templates/steps-update-winappsdk-and-restore-nuget.yml`
to ensure the restore process operates from the correct directory.
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## PR Checklist

- [ ] Closes: #xxx
<!-- - [ ] Closes: #yyy (add separate lines for additional resolved
issues) -->
- [ ] **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

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed

.pipelines/v2/templates/steps-update-winappsdk-and-restore-nuget.yml

@@ -35,3 +35,4 @@ steps:
     projects: '$(build.sourcesdirectory)\**\*.slnx'
     feedsToUse: 'config'
     nugetConfigPath: '$(build.sourcesdirectory)\nuget.config'
+    workingDirectory: '$(build.sourcesdirectory)'