Which default Linux distribution does Travis CI apply when the `dist` key is omitted in `.travis.yml`?
The goal is to guarantee repeatable builds by understanding which base image Travis CI selects when a `.travis.yml` file does not explicitly declare a `dist` value. Travis CI’s default distribution is not fixed; it has shifted from Trusty to Xenial to Focal and may vary depending on the age of the account or internal updates, making builds non‑repeatable if
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