Use x64 version of desktop msbuild if process arch is amd64

[ViktorHofer]

Follow-up to #7531

I looked at a few binlogs from a desktop msbuild invocation today and was surprised that we still launch the x86 version of msbuild.exe by default.

Looking at

arcade/eng/common/tools.ps1

Lines 469 to 473 in b7be18a

	$local:Prefer64bit = if (Get-Member -InputObject $vsRequirements -Name 'Prefer64bit') { $vsRequirements.Prefer64bit } else { $false }
	if ($local:Prefer64bit -and (Test-Path(Join-Path $local:BinFolder "amd64"))) {
	$global:_MSBuildExe = Join-Path $local:BinFolder "amd64\msbuild.exe"
	} else {
	$global:_MSBuildExe = Join-Path $local:BinFolder "msbuild.exe"

, the opt-in never got switched to a default.