use curl instead of ps: Start-FileDownload

appveyor.yml

@@ -38,7 +38,7 @@ clone_folder: c:\gopath\src\github.com\hashicorp\packer
 
 # Install needed build dependencies
 install:
-  - ps: Start-FileDownload "https://cygwin.com/$env:CYG_SETUP" -FileName "$env:CYG_SETUP"
+  - curl -fsS -o %CYG_SETUP% "https://cygwin.com/%CYG_SETUP%"
   - '%CYG_SETUP% --quiet-mode --no-shortcuts --only-site --root "%CYG_ROOT%" --site "%CYG_MIRROR%" --local-package-dir "%CYG_CACHE%" --packages "%CYG_PACKAGES%" --upgrade-also'
   - echo %Path%
   - go version