Add -cross flag to build.ps1 (dotnet#100678)

Fixes VMR build error on Windows after dotnet/installer#19321

We haven't used or passed this on Windows before and a bunch of logic is conditioned based on the `CrossBuild` property (which is what `-cross` is turned into in build.sh) so ignore it for now.

eng/build.ps1

@@ -12,6 +12,7 @@ Param(
   [string]$testscope,
   [switch]$testnobuild,
   [ValidateSet("x86","x64","arm","arm64","wasm")][string[]][Alias('a')]$arch = @([System.Runtime.InteropServices.RuntimeInformation]::ProcessArchitecture.ToString().ToLowerInvariant()),
+  [switch]$cross = $false,
   [string][Alias('s')]$subset,
   [ValidateSet("Debug","Release","Checked")][string][Alias('rc')]$runtimeConfiguration,
   [ValidateSet("Debug","Release")][string][Alias('lc')]$librariesConfiguration,