ci: update code_coverage workflow to use rust stable version

Author: notmandatory

.github/workflows/code_coverage.yml

@@ -19,7 +19,7 @@ jobs:
       - name: Install Rust toolchain
         uses: actions-rs/toolchain@v1
         with:
-            toolchain: "1.67.0"
+            toolchain: stable
             override: true
             profile: minimal
             components: llvm-tools-preview