Fix CI on Windows

Author: alexcrichton

.github/workflows/main.yml

@@ -71,7 +71,7 @@ jobs:
       with:
         submodules: true
     - name: Install Rust (rustup)
-      run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
+      run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
       if: matrix.os != 'macos-latest'
     - name: Install Rust (macos)
       run: |