package/ci: cache downloaded files on AppVeyor.

Hopefully resolves the timeout errors.

package/ci/appveyor.yml

@@ -23,7 +23,7 @@ install:
 - cinst ninja
 
 # OpenAL
-- appveyor DownloadFile http://kcat.strangesoft.net/openal-binaries/openal-soft-1.17.2-bin.zip
+- IF NOT EXIST %APPVEYOR_BUILD_FOLDER%\openal-soft-1.17.2-bin.zip appveyor DownloadFile http://kcat.strangesoft.net/openal-binaries/openal-soft-1.17.2-bin.zip
 - 7z x openal-soft-1.17.2-bin.zip
 - ren openal-soft-1.17.2-bin openal
 - ren openal\bin\Win32\soft_oal.dll OpenAL32.dll
@@ -31,7 +31,7 @@ install:
 - echo drivers=null >> %APPDATA%/alsoft.ini
 
 # SDL2
-- appveyor DownloadFile http://www.libsdl.org/release/SDL2-devel-2.0.3-VC.zip
+- IF NOT EXIST %APPVEYOR_BUILD_FOLDER%\SDL2-devel-2.0.3-VC.zip appveyor DownloadFile http://www.libsdl.org/release/SDL2-devel-2.0.3-VC.zip
 - 7z x SDL2-devel-2.0.3-VC.zip
 - ren SDL2-2.0.3 SDL
 
@@ -46,6 +46,10 @@ install:
 - cd ..
 - cd ..
 
+cache:
+- SDL2-devel-2.0.3-VC.zip
+- openal-soft-1.17.2-bin.zip
+
 build_script:
 - mkdir build
 - cd build