[bazel,ci] add script to test airgapped bazel builds
[bazel,ci] add script to test airgapped bazel builds

This adds a script that configures a network namespace to isolate a
bazel build process from the Internet to provide a blocking CI test to
ensure our Bazel configuration works in an airgapped environment.

Signed-off-by: Timothy Trippel <77eacc84f9735d3cbb55a271710be269fc3a5818@google.com>

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