appveyor.yml: Cache NASM binary package

... since www.nasm.us seems to be down frequently.  This doesn't help us
at the moment, but hopefully once the site is back up this will prevent
future build failures.

appveyor.yml

@@ -1,10 +1,10 @@
 install:
   - cmd: >-
-      mkdir c:\installers
+      if not exist c:\installers mkdir c:\installers
 
       mkdir c:\temp
 
-      curl -fSL -o c:\installers\nasm-2.10.01-win32.zip http://www.nasm.us/pub/nasm/releasebuilds/2.10.01/win32/nasm-2.10.01-win32.zip
+      if not exist c:\installers\nasm-2.10.01-win32.zip curl -fSL -o c:\installers\nasm-2.10.01-win32.zip http://www.nasm.us/pub/nasm/releasebuilds/2.10.01/win32/nasm-2.10.01-win32.zip
 
       7z x c:\installers\nasm-2.10.01-win32.zip -oc:\ > c:\installers\nasm.install.log
 
@@ -22,6 +22,9 @@ install:
 
       git clone --depth=1 https://github.com/libjpeg-turbo/buildscripts.git -b %APPVEYOR_REPO_BRANCH% c:/buildscripts
 
+cache:
+  - c:\installers\nasm-2.10.01-win32.zip -> appveyor.yml
+
 build_script:
   - cmd: >-
       for /f %%i in ('"cygpath %CD%"') do set MINGWPATH=%%i