Restore InitializeToolset call in MSBuild function

When the Arcade msbuild logger was removed in #16814, the
InitializeToolset call in the MSBuild function (tools.{sh,ps1}) was
removed alongside it. That call had a load-bearing side effect:
InitializeToolset invokes GetNuGetPackageCachePath, which exports
NUGET_PACKAGES.

Without NUGET_PACKAGES exported, NuGet restore defaults to the user
profile while RepoLayout.props sets MSBuild's $(NuGetPackageRoot) to
$(RepoRoot)/.packages/ under ContinuousIntegrationBuild=true. Generated
.nuget.g.props imports guarded by Exists($(NuGetPackageRoot)...) are
silently skipped, and properties contributed by them (e.g.
XunitConsoleNetCoreAppPath) end up undefined.

Restore the InitializeToolset call in the MSBuild function gated on
$ci, matching the pre-#16814 behavior. Placing it in the MSBuild
function rather than at tools.{sh,ps1} load time ensures any
configure-toolset overrides have already been imported.

Fixes #16898

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: akoeplinger

eng/common/tools.ps1

@@ -748,6 +748,8 @@ function Stop-Processes() {
 #
 function MSBuild() {
   if ($ci) {
+    InitializeToolset | Out-Null
+
     $env:NUGET_PLUGIN_HANDSHAKE_TIMEOUT_IN_SECONDS = 20
     $env:NUGET_PLUGIN_REQUEST_TIMEOUT_IN_SECONDS = 20
     Write-PipelineSetVariable -Name 'NUGET_PLUGIN_HANDSHAKE_TIMEOUT_IN_SECONDS' -Value '20'

eng/common/tools.sh

@@ -505,6 +505,8 @@ function MSBuild {
   local args=( "$@" )
 
   if [[ "$ci" == true ]]; then
+    InitializeToolset
+
     export NUGET_PLUGIN_HANDSHAKE_TIMEOUT_IN_SECONDS=20
     export NUGET_PLUGIN_REQUEST_TIMEOUT_IN_SECONDS=20
     Write-PipelineSetVariable -name "NUGET_PLUGIN_HANDSHAKE_TIMEOUT_IN_SECONDS" -value "20"