Rename TRAVIS env var to CI

This env var, used to skip some tests, was renamed to a more generic name. See https://github.com/pexpect/pexpect/pull/743.

Without this fix these tests fail again during autopkgtest.

Fixed autopkgtests:

• https://autopkgtest.ubuntu.com/results/autopkgtest-noble-upils-test-ppa/noble/amd64/p/pexpect/20240212_102715_a873b@/log.gz
• https://autopkgtest.ubuntu.com/results/autopkgtest-noble-upils-test-ppa/noble/arm64/p/pexpect/20240212_103005_44dab@/log.gz
• https://autopkgtest.ubuntu.com/results/autopkgtest-noble-upils-test-ppa/noble/s390x/p/pexpect/20240212_111956_17d92@/log.gz
• https://autopkgtest.ubuntu.com/results/autopkgtest-noble-upils-test-ppa/noble/ppc64el/p/pexpect/20240215_090229_a0e0a@/log.gz