test UWP on GNU targets

Author: newpavlov

appveyor.yml

@@ -22,9 +22,9 @@ environment:
   matrix:
   - TARGET: x86_64-pc-windows-msvc
   - TARGET: i686-pc-windows-msvc
-  - TARGET: x86_64-uwp-windows-msvc
+  - TARGET: x86_64-uwp-windows-gnu
     CHANNEL: nightly
-  - TARGET: i686-uwp-windows-msvc
+  - TARGET: i686-uwp-windows-gnu
     CHANNEL: nightly
 install:
   - appveyor DownloadFile https://win.rustup.rs/ -FileName rustup-init.exe