Pin Windows CI to windows-2025 runner

windows-latest now resolves to the VS2026 image; pin the Win32 and UWP jobs
to windows-2025 explicitly so the toolset matches what the GSL v4.2.2 bump in
this PR is validated against and the image can't silently drift again.

Author: Branimir Karadžić

.github/workflows/build-uwp.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   build:
-    runs-on: windows-latest
+    runs-on: windows-2025
     timeout-minutes: 15
     steps:
       - uses: actions/checkout@v5

.github/workflows/build-win32.yml

@@ -12,7 +12,7 @@ on:
 
 jobs:
   build:
-    runs-on: windows-latest
+    runs-on: windows-2025
     timeout-minutes: 15
     steps:
       - uses: actions/checkout@v5