[C++/CI] Activate flight build in ci/docker_build_cpp.sh

[asfimport]

This is currently not build.

Reporter: Uwe Korn / @xhochy
Assignee: Uwe Korn / @xhochy

Related issues:

• [C++] Rework CMake third-party logic (is a child of)

_{Note: This issue was originally created as ARROW-4614. Please see the migration documentation for further details.}

[asfimport]

Uwe Korn / @xhochy:
Resolved by PR #3688