Always use the Rust version in package names
The format of the tarballs produced by CI is roughly the following:
{component}-{release}-{target}.{ext}
While on the beta and nightly channels `{release}` is just the channel name, on the stable channel is either the Rust version or the version of the component we're shipping:
cargo-0.47.0-x86_64-unknown-linux-gnu.tar.xz
clippy-0.0.212-x86_64-unknown-linux-gnu.tar.xz
llvm-tools-1.46.0-x86_64-unknown-linux-gnu.tar.xz
miri-0.1.0-x86_64-unknown-linux-gnu.tar.xz
rls-1.41.0-x86_64-unknown-linux-gnu.tar.xz
rust-1.46.0-x86_64-unknown-linux-gnu.tar.xz
...
This makes it really hard to get the package URL without having access to the manifest (and there is no manifest on ci-artifacts.rlo), as there is no consistent version number to use.
This PR addresses the problem by always using the Rust version number as `{release}` for the stable channel, regardless of the version number of the component we're shipping. I chose that instead of "stable" to avoid breaking the URL scheme *that* much.
Rustup should not be affected by this change, as it fetches the URLs from the manifest. Unfortunately we don't have a way to test other clients before making a stable release, as this change only affects the stable channel.
r? `@Mark-Simulacrum`
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