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MINOR: [CI][C++] Add C++ example builds to "cpp" Crossbow task group (#43975)

### Rationale for this change

The `python` task group already includes the Python example builds. This PR does the same for the `cpp` task group.

### Are these changes tested?

By CI itself.

### Are there any user-facing changes?

No.

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>