fix: add shell: bash to negative smoke checks for cross-platform portability

The 'Invalid command fails' and 'Invalid port fails' smoke test steps use
bash if/then/fi syntax that fails on Windows runners (default pwsh shell).
Explicitly setting shell: bash makes these steps portable across the
Windows/Linux/macOS CI matrix.

.github/workflows/ci.yml

@@ -104,12 +104,14 @@ jobs:
       - name: rustunnel generate --help
         run: cargo run --release -- generate --help
       - name: Invalid command fails
+        shell: bash
         run: |
           if cargo run --release -- invalid-cmd 2>&1; then
             echo "ERROR: invalid command should have failed"
             exit 1
           fi
       - name: Invalid port fails
+        shell: bash
         run: |
           if cargo run --release -- listen --listen localhost:bad --cert x --key x --ca-cert x --auth-token x 2>&1; then
             echo "ERROR: invalid port should have failed"