Use simple connectivity check for startup health probe

Older spiceio binaries may not set the Server header at all. The startup
health check only needs to confirm the process we just started is
responding — drop the Server header requirement and use a plain GET.

Author: lukekim

.github/actions/setup/action.yml

@@ -160,7 +160,7 @@ runs:
             exit 1
           fi
           ENDPOINT=$(grep 'listening on' "$SPICEIO_LOG" 2>/dev/null | grep -o 'http://[^ ]*' | tail -1 || true)
-          if [[ -n "$ENDPOINT" ]] && curl -s -I "$ENDPOINT/" 2>/dev/null | grep -qi "server: spiceio"; then
+          if [[ -n "$ENDPOINT" ]] && curl -s -o /dev/null "$ENDPOINT/" 2>/dev/null; then
             echo "spiceio ready at $ENDPOINT (PID $PID)"
             echo "endpoint=$ENDPOINT" >> "$GITHUB_OUTPUT"
             exit 0