build: Some minor improvements to appveyor

* Add cache support
* Finish all build jobs if one failed

Author: tesuji

appveyor.yml

@@ -15,7 +15,6 @@ environment:
   RUSTFLAGS: -Ctarget-feature=+crt-static
   RUST_BACKTRACE: 1
   matrix:
-    fast_finish: true
   - TARGET: x86_64-pc-windows-msvc
     NO_ADD: 1
     ALLOW_PR: 1