Enable test website on x86 (#12913)

## Description
Enable deploying the test website on x86 CI environments.

### Type of Change
- Bug fix (non-breaking change which fixes an issue)

### Why
Custom MSBuild logic causes `PlatformTarget` to yield the value `x64` for `dotnet` commands even when the solution was built for `x86`.

This causes IIS not finding the website location at `vnext\target\$(BuildPlatform)\$(BuildConfiguration)...` when `BuildPlatform` is not equal to `x64` and returning HTTP `40x` status errors.

### What
Make IIS use a fixed value for the `BuildPlatform` in the website's location.

## Testing
Temporarily enabled `x86|Release` in the pull request matrix to ensure the website works as expected.
Disabled before merging.
See https://dev.azure.com/ms/react-native-windows/_build/results?buildId=560987&view=logs&jobId=9424bcb6-bf6e-5e05-97df-1709db307089&j=9424bcb6-bf6e-5e05-97df-1709db307089&t=06e5a27c-ef2d-5a24-8119-31e1f439c522

.ado/jobs/desktop.yml

@@ -245,7 +245,9 @@ jobs:
                   SSLCertThumbPrint: $(TestWebsiteCertificateThumbprint)
                   # IIS Website
                   WebsiteName: RNW Test Website
-                  WebsitePhysicalPath: $(Build.SourcesDirectory)\vnext\target\${{ matrix.BuildPlatform }}\${{ matrix.BuildConfiguration }}\Microsoft.ReactNative.Test.Website\Publish
+                  # Hard-coding x64 for publish path.
+                  # Our MSBuild customizations cause dotnet commans to assume the default PlatformTarget
+                  WebsitePhysicalPath: $(Build.SourcesDirectory)\vnext\target\x64\${{ matrix.BuildConfiguration }}\Microsoft.ReactNative.Test.Website\Publish
                   WebsitePhysicalPathAuth: WebsiteUserPassThrough
                   CreateOrUpdateAppPoolForWebsite: false
                   ConfigureAuthenticationForWebsite: false