fix AppVeyor MIRI_SYSROOT

Author: RalfJung

appveyor.yml

@@ -32,7 +32,7 @@ test_script:
     - cargo test --release --all-features
     # Get ourselves a MIR-full libstd, and use it henceforth
     - cargo run --release --all-features --bin cargo-miri -- miri setup
-    - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\miri
+    - set MIRI_SYSROOT=%USERPROFILE%\AppData\Local\miri\miri\cache
     # Test miri with full MIR
     - cargo test --release --all-features