.appveyor.yml: Update Rust version

See comments for 21b2e418e05c (".appveyor.yml: Update Rust version").

Signed-off-by: Quentin Monnet <qmo@qmon.net>

.appveyor.yml

@@ -12,7 +12,7 @@ platform:
 environment:
   matrix:
     - TOOLCHAIN_VERSION: 14.0
-      RUST: 1.76.0
+      RUST: 1.81.0
     - TOOLCHAIN_VERSION: 14.0
       RUST: beta
     - TOOLCHAIN_VERSION: 14.0