ci: fix bird package name (alpine ships 'bird', not 'bird2')

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -21,7 +21,7 @@ RUN CGO_ENABLED=0 go build -trimpath \
         -o /out/flock-installer ./cmd/flock-installer
 
 FROM alpine:3.21
-RUN apk add --no-cache iproute2 bird2 ca-certificates
+RUN apk add --no-cache iproute2 bird ca-certificates
 COPY --from=build /out/flock           /usr/local/bin/flock
 COPY --from=build /out/flock-agent     /usr/local/bin/flock-agent
 COPY --from=build /out/flock-installer /usr/local/bin/flock-installer