Fix Windows CI for now

.github/workflows/main.yml

@@ -57,7 +57,7 @@ jobs:
     steps:
     - uses: actions/checkout@master
     - 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: |
@@ -91,7 +91,7 @@ jobs:
     steps:
     - uses: actions/checkout@master
     - name: Install Rust
-      run: rustup update 1.31.0 && rustup default 1.31.0
+      run: rustup update 1.31.0 --no-self-update && rustup default 1.31.0
     - run: cargo build
 
   rustfmt: