Roll back windows build image to 2019 on android build job

Per workaround suggested in
actions/runner-images#11402 (comment).

Applying this now as my hopes for a swift resolution without changes on
our side are slim to none (read thread linked above in full to learn
why).

.github/workflows/ci.yml

@@ -96,7 +96,7 @@ jobs:
 
   build-only-android:
     name: Build only (Android)
-    runs-on: windows-latest
+    runs-on: windows-2019
     timeout-minutes: 60
     steps:
       - name: Checkout