.appveyor.yml: Update Rust version

I'd like to avoid pinning Rust to a specific version in Appveyor. This
is less deterministic for the tests, but because we don't run them often
anyway we want to use up-to-date versions when we do. And this would be
consistent with the Linux workflows.

However, I haven't found a way to pass a "stable" channel to the
workflow, if using "stable" it fails to download the computed .msi name
(as [0]) which doesn't exist. Looking at the download page [1], there's
indeed no such channel name, and packages for the stable releases use a
version number.

So we just bump the version to the current Rust stable in this commit.

[0] https://static.rust-lang.org/dist/rust-stable-x86_64-pc-windows-msvc.msi
[1] https://forge.rust-lang.org/infra/other-installation-methods.html

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

.appveyor.yml

@@ -12,7 +12,7 @@ platform:
 environment:
   matrix:
     - TOOLCHAIN_VERSION: 14.0
-      RUST: 1.64.0
+      RUST: 1.76.0
     - TOOLCHAIN_VERSION: 14.0
       RUST: beta
     - TOOLCHAIN_VERSION: 14.0