external_integration_test: use serve_file
external_integration_test: use serve_file

...instead of directly calling nc with the `pick_random_unused_tcp_port` source
of flakyness.

Works towards #7906.

Change-Id: I5eefa4badf18f002e34a184a1c14fea5bf61e51a
PiperOrigin-RevId: 241924579

dslomov/bazel-windows,ButterflyNetwork/bazel,perezd/bazel,aehlig/bazel,dslomov/bazel,meteorcloudy/bazel,ulfjack/bazel,aehlig/bazel,dslomov/bazel-windows,katre/bazel,cushon/bazel,safarmer/bazel,perezd/bazel,akira-baruah/bazel,safarmer/bazel,dslomov/bazel-windows,werkt/bazel,davidzchen/bazel,dslomov/bazel-windows,ulfjack/bazel,twitter-forks/bazel,ButterflyNetwork/bazel,dslomov/bazel-windows,bazelbuild/bazel,aehlig/bazel,katre/bazel,meteorcloudy/bazel,meteorcloudy/bazel,bazelbuild/bazel,cushon/bazel,werkt/bazel,twitter-forks/bazel,aehlig/bazel,dslomov/bazel,perezd/bazel,dslomov/bazel,ButterflyNetwork/bazel,katre/bazel,meteorcloudy/bazel,akira-baruah/bazel,perezd/bazel,twitter-forks/bazel,davidzchen/bazel,cushon/bazel,ulfjack/bazel,dslomov/bazel,safarmer/bazel,katre/bazel,davidzchen/bazel,werkt/bazel,twitter-forks/bazel,akira-baruah/bazel,cushon/bazel,bazelbuild/bazel,bazelbuild/bazel,bazelbuild/bazel,ButterflyNetwork/bazel,dslomov/bazel-windows,dslomov/bazel,davidzchen/bazel,werkt/bazel,ulfjack/bazel,akira-baruah/bazel,safarmer/bazel,safarmer/bazel,davidzchen/bazel,perezd/bazel,safarmer/bazel,aehlig/bazel,twitter-forks/bazel,ulfjack/bazel,meteorcloudy/bazel,davidzchen/bazel,werkt/bazel,twitter-forks/bazel,katre/bazel,akira-baruah/bazel,meteorcloudy/bazel,cushon/bazel,cushon/bazel,ulfjack/bazel,ButterflyNetwork/bazel,davidzchen/bazel,aehlig/bazel,perezd/bazel,akira-baruah/bazel,ButterflyNetwork/bazel,ulfjack/bazel,meteorcloudy/bazel,bazelbuild/bazel,dslomov/bazel,twitter-forks/bazel,werkt/bazel,perezd/bazel,katre/bazel,dslomov/bazel,aehlig/bazel