Use separate caches for zlib and libpng on Windows 2022 vs 2025

Windows 2025 no longer has a D:/ drive
<https://github.com/actions/runner-images/issues/12416>
which affects where the cached files go.

This can result in one of two errors in the "Install zlib" step
after restoring from cache:

- file cannot create directory: D:/a/rgbds/rgbds/install_dir/lib.
  Maybe need administrative privileges.
- file INSTALL cannot find "C:/a/rgbds/rgbds/zbuild/Release/zlib1.dll":
  File exists.

Separate caches per OS should solve this problem.

.github/workflows/testing.yml

@@ -149,7 +149,7 @@ jobs:
           path: |
             zbuild
             pngbuild
-          key: ${{ matrix.arch }}-${{ hashFiles('zlib/**', 'libpng/**') }}
+          key: ${{ matrix.os }}-${{ matrix.arch }}-${{ hashFiles('zlib/**', 'libpng/**') }}
       - name: Build zlib
         if: steps.cache.outputs.cache-hit != 'true'
         shell: bash