ci: install rustup via official script (not pre-installed in image)

The catthehacker/ubuntu:act-latest image does not have rustup in PATH
('rustup: command not found', exit 127). Replace the direct rustup
invocation with the official install script from sh.rustup.rs, which
installs rustup + the stable toolchain in one step.

Also adds ~/.cargo/bin to GITHUB_PATH so subsequent steps (cargo clippy,
cargo build, cargo test, cargo audit) can find cargo/rustc.

.github/workflows/ci.yml

@@ -34,7 +34,9 @@ jobs:
       - uses: https://gitea.com/actions/checkout@v4
 
       - name: Install Rust toolchain (stable)
-        run: rustup toolchain install stable --profile minimal --component clippy && rustup default stable
+        run: |
+          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"
 
       - name: Install system dependencies
         run: |