.appveyor.yml: Update Rust toolchain to 1.64

1.20 is an old version, let's use the current stable instead.

.appveyor.yml

@@ -9,7 +9,7 @@ platform:
 environment:
   matrix:
     - TOOLCHAIN_VERSION: 14.0
-      RUST: 1.20.0
+      RUST: 1.64.0
     - TOOLCHAIN_VERSION: 14.0
       RUST: beta
     - TOOLCHAIN_VERSION: 14.0