Move away from the legacy windevbuildagents pool (#13218)

There is an internal requirement that we move to build agents that we
don't run ourselves. This discharges us of that requirement!

We're switching between the WinDevPool pool and the WinDevPoolOSS pool
based on whether this code is being built in the open-source tenant or
the internal/private one.

.pipelines/ci/templates/build-powertoys-ci.yml

@@ -9,7 +9,11 @@ jobs:
   variables:
     BuildConfiguration: ${{ parameters.configuration }}
     BuildPlatform: ${{ parameters.platform }}
-  pool: "windevbuildagents"
+  pool:
+    ${{ if eq(variables['System.CollectionUri'], 'https://dev.azure.com/ms/') }}:
+      name: WinDevPoolOSS-L
+    ${{ if ne(variables['System.CollectionUri'], 'https://dev.azure.com/ms/') }}:
+      name: WinDevPool-L
   timeoutInMinutes: 120
   strategy:
     maxParallel: 10