ci(appveyor): use proxy to solve SSL error

Thanks to badrelmers.
rust-lang/cargo#12296 (comment)

Author: dbohdan

appveyor.yml

@@ -8,9 +8,15 @@ cache:
   - '$(CARGO_PATH) -> appveyor.yml, Cargo.lock, Cargo.toml'
 
 install:
+  - cmd: appveyor-retry appveyor DownloadFile https://github.com/projectdiscovery/proxify/releases/download/v0.0.9/proxify_0.0.9_windows_386.zip -FileName proxify.zip
+  - cmd: unzip -n proxify.zip
+  - ps: $Proxify = Start-Process -FilePath proxify.exe -ArgumentList "-config","proxify-config","-http-addr","127.0.0.1:18080" -PassThru
+  - cmd: certutil -addstore -f -user "Root" "proxify-config\cacert.pem"
   - cmd: appveyor-retry appveyor DownloadFile https://win.rustup.rs -FileName rustup-init.exe
-  - cmd: rustup-init.exe --default-toolchain %RUST_VERSION% -y
+  - cmd: set HTTP_PROXY=127.0.0.1:18080
+  - cmd: set HTTPS_PROXY=127.0.0.1:18080
   - cmd: set PATH=%CARGO_PATH%\bin;%PATH%
+  - cmd: rustup-init.exe --default-toolchain %RUST_VERSION% -y
 
 build_script:
   - cmd: cargo build
@@ -22,5 +28,8 @@ test_script:
 after_test:
   - cmd: copy %S2PNG_COMMAND% s2png-win32.exe
 
+on_finish:
+  - ps: Stop-Process -Id $Proxify.Id
+
 artifacts:
   - path: s2png-win32.exe