Fix CI failure on Windows

--no-self-update is necessary because the windows environment cannot
self-update rustup.exe.

Author: taiki-e

.github/workflows/ci.yml

@@ -44,7 +44,8 @@ jobs:
     steps:
       - uses: actions/checkout@master
       - name: Install Rust
-        run: rustup update ${{ matrix.rust }} && rustup default ${{ matrix.rust }}
+        # --no-self-update is necessary because the windows environment cannot self-update rustup.exe.
+        run: rustup update ${{ matrix.rust }} --no-self-update && rustup default ${{ matrix.rust }}
       - name: Test
         run: ./ci/${{ matrix.crates }}.sh