ci: support parallel cmake build in `test-jepsen`

In order to support parallel cmake build in the `test-jepsen` target
we need to run Jepsen testing in a `debian-buster` container instead
of `debian-stretch` due to the old `cmake` version in it. The `cmake`
utility started to support `--parallel` option since 3.12 version [1].

In Debian Stretch `cmake` has version 3.7.
In Debian Buster `cmake` has version 3.13.

[1] https://cmake.org/cmake/help/latest/release/3.12.html#command-line

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