ci(appveyor): use proxy to solve SSL error

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

appveyor.yml

@@ -2,12 +2,18 @@ environment:
   CARGO_PATH: c:\users\appveyor\.cargo
   RUST_VERSION: 1.43.0
   CARGO_HTTP_CHECK_REVOKE: false
+  HTTP_PROXY: 127.0.0.1:18080
+  HTTPS_PROXY: 127.0.0.1:18080
   S2PNG_COMMAND: target\debug\s2png.exe
 
 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 proxify.zip
+  - cmd: proxify.exe -http-addr 127.0.0.1:18080
+  - cmd: certutil -addstore -f -user "Root" "%USERPROFILE%\.config\proxify\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 PATH=%CARGO_PATH%\bin;%PATH%