CI: Make the Windows CI builds use separate caches

Looking at recent build failures, one of Debug or Release is using the prebuilt
vcpkg from the cache, and the other one is detecting it but then deciding to
rebuild many of the packages.

The build logs circa 8bd8c53f98 are no longer
accessible, only the success/failure and time taken are still accessible.
Looking at build timings around those changes is inconclusive, however my guess
is that the CI is noticing that CFG is different between the two builds, and
that's forcing a rebuild for one of them.

.github/workflows/ci-main.yml

@@ -267,7 +267,7 @@ jobs:
           path: |
             D:/a/wesnoth/vcpkg
             D:/a/wesnoth/wesnoth/vcpkg_installed
-          key: win-cache-master-0011
+          key: win-cache-master-${{ matrix.cfg }}-0012
 
       - name: Add msbuild to PATH
         uses: microsoft/setup-msbuild@v1.1.3