[ci] Update 'test-readme-examples' job to use nightly-2021-03-23

Author: notmandatory

.github/workflows/cont_integration.yml

@@ -65,7 +65,7 @@ jobs:
             target
           key: ${{ runner.os }}-cargo-test-md-docs-${{ hashFiles('**/Cargo.toml','**/Cargo.lock') }}
       - name: Set default toolchain
-        run: rustup default nightly
+        run: rustup default nightly-2021-03-23
       - name: Set profile
         run: rustup set profile minimal
       - name: Update toolchain