ci: use rsproxy.cn for rustup component downloads

rustup components (~60MB) from static.rust-lang.org are slow even
through proxy. Switch to rsproxy.cn/rustup mirror (domestic, direct
connection via NO_PROXY).

.github/workflows/ci.yml

@@ -35,6 +35,8 @@ jobs:
 
       - name: Install Rust toolchain (stable)
         run: |
+          export RUSTUP_DIST_SERVER=https://rsproxy.cn/rustup
+          export RUSTUP_UPDATE_ROOT=https://rsproxy.cn/rustup/rustup
           curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --profile minimal --component clippy
           echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
 
@@ -104,7 +106,9 @@ jobs:
       - uses: https://gitea.com/actions/checkout@v4
 
       - name: Install Rust toolchain (stable)
-        run: rustup toolchain install stable --profile minimal && rustup default stable
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain stable --profile minimal
+          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
 
       - name: Configure cargo mirror (CI-only)
         run: |