ARROW-11406: [CI][C++] Fix ccache caching on Travis-CI
ARROW-11406: [CI][C++] Fix ccache caching on Travis-CI

Closes #9344 from pitrou/ARROW-11406-travis-docker-ccache

Authored-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>

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