CI: we now need the nightly toolchain as well

src/tools/miri/.github/workflows/setup/action.yml

@@ -35,6 +35,10 @@ runs:
         run: cargo install -f rustup-toolchain-install-master hyperfine
         shell: bash
 
+      - name: Install nightly toolchain
+        run: rustup toolchain install nightly --profile minimal
+        shell: bash
+
       - name: Install "master" toolchain
         run: |
           if [[ ${{ github.event_name }} == 'schedule' ]]; then