Simplify build matrix to focus on Android platform

Removed Windows and macOS configurations from `build.yml`, leaving only Android-specific settings. Updated `vcpkg_triplet` and `vcpkg_host_triplet` to `arm64-android`. Set `android_abi` to `arm64-v8a` and `android_api` to `21`. Standardized `check_disk_space` to `df -h`. Retained relevant environment variables for Android builds.

Author: JoergAtGithub

.github/workflows/build.yml

@@ -23,24 +23,6 @@ jobs:
             check_disk_space: df -h
             android_abi: arm64-v8a
             android_api: 21
-          - os: windows-2022
-            vcpkg_path: C:\mixxx-vcpkg
-            vcpkg_bootstrap: .\bootstrap-vcpkg.bat
-            vcpkg_triplet: x64-windows
-            vcpkg_host_triplet: x64-windows           
-            check_disk_space: Get-PSDrive
-          - os: macos-13
-            vcpkg_path: /Users/runner/mixxx-vcpkg
-            vcpkg_bootstrap: ./bootstrap-vcpkg.sh
-            vcpkg_triplet: x64-osx-min1100
-            vcpkg_host_triplet: x64-osx-min1100
-            check_disk_space: df -h
-          - os: macos-13
-            vcpkg_path: /Users/runner/mixxx-vcpkg
-            vcpkg_bootstrap: ./bootstrap-vcpkg.sh
-            vcpkg_triplet: arm64-osx-min1100-release
-            vcpkg_host_triplet: x64-osx-min1100-release
-            check_disk_space: df -h
     env:
       VCPKG_DEFAULT_TRIPLET: ${{ matrix.vcpkg_triplet }}
       VCPKG_DEFAULT_HOST_TRIPLET: ${{ matrix.vcpkg_host_triplet }}