workflows: rust: Build docs with stable toolchain

This works around issue #74134 in rustdoc and should be reverted when PR #74147
is merged and available in a nightly.

 - #74134: rust-lang/rust#74134
 - #74147: rust-lang/rust#74147

Author: dennis-hamester

.github/workflows/rust.yaml

@@ -48,10 +48,10 @@ jobs:
     steps:
       - name: Checkout
         uses: actions/checkout@v2
-      - name: Install Rust nightly
+      - name: Install Rust stable
         uses: actions-rs/toolchain@v1
         with:
-          toolchain: nightly
+          toolchain: stable
           profile: minimal
           default: true
       - name: Rust version