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ARROW-9262: [Packaging][Linux][CI] Use Ubuntu 18.04 to build ARM64 packages on Travis CI

We got the following error with Ubuntu 20.04:

    gpg: Fatal: can't disable core dumps: Operation not permitted

Closes #7573 from kou/linux-packages-use-bionic

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>