Workaround SourceForget brokenness in AppVeyor builds

appveyor.yml

@@ -39,6 +39,9 @@ init:
 
 # scripts that run after cloning repository (before the build step, not after!)
 install:
+  # temporarily download NSIS archive ourselves, until sourceforge is back up
+  - ps: New-Item -ItemType Directory -Force -Path $env:TEMP\chocolatey\nsis.portable\3.02.0.20160720
+  - ps: invoke-webrequest -uri https://10gbps-io.dl.sourceforge.net/project/nsis/NSIS%203%20Pre-release/3.0rc2/nsis-3.0rc2.zip -outfile $env:TEMP\chocolatey\nsis.portable\3.02.0.20160720\download
   # test_bbackupd needs 7zip (or cmake -E tar) to extract tar archives on Windows:
   - cinst -y --limit-output 7zip.commandline nsis.portable
   - dir "c:\Program Files"
@@ -107,3 +110,5 @@ deploy:
         - master
         - windows_binary_packages
 
+cache:
+  - '%TEMP%\chocolatey'