.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>
Quentin Monnet