fix missing description

Github documentation states that it is a required field and my linter complained

https://docs.github.com/en/actions/reference/workflows-and-actions/metadata-syntax#description

.github/actions/setup-rust/action.yml

@@ -1,4 +1,7 @@
 name: 'Setup Rust'
+description: |
+  Setup the rust toolchain and environment for the selected toolchain
+
 inputs:
   rust-version:
     required: true