Appveyor: enable build cache and better download command

appveyor.yml

@@ -13,9 +13,11 @@ init:
 - ver
 
 install:
-- cd C:\
-- certutil -urlcache -split -f "https://s3-us-west-2.amazonaws.com/px4-tools/PX4+Windows+Cygwin+Toolchain/PX4+Windows+Cygwin+Toolchain+0.3.msi" Toolchain.msi
-- msiexec /i Toolchain.msi /quiet /qn /norestart /log install.log
+- ps: if (-not (Test-Path C:\Toolchain.msi)) {Invoke-WebRequest https://s3-us-west-2.amazonaws.com/px4-tools/PX4+Windows+Cygwin+Toolchain/PX4+Windows+Cygwin+Toolchain+0.3.msi -OutFile C:\Toolchain.msi}
+- start /wait msiexec /i C:\Toolchain.msi /quiet /qn /norestart /log C:\install.log
 
 build_script:
 - call C:\PX4\toolchain\clone-px4-build-sitl.bat
+
+cache:
+- C:\Toolchain.msi