ARROW-15078: [C++] Silence CMake error "includes non-existent path" with bundled OpenTelemetry

[mbrobbel]

This prevents:

CMake Error in src/arrow/CMakeLists.txt:
  Imported target "opentelemetry-cpp::trace" includes non-existent path

    "<path>/build/opentelemetry_ep-install/include"

  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:

  * The path was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and references files it does not
  provide.

[github-actions]

https://issues.apache.org/jira/browse/ARROW-15078

[lidavidm]

Thanks for fixing this!

[ursabot]

Benchmark runs are scheduled for baseline = 575e2ff and contender = 6b31026. 6b31026 is a master commit associated with this PR. Results will be available as each benchmark for each run completes.
Conbench compare runs links:
[Finished ⬇️0.0% ⬆️0.0%] ec2-t3-xlarge-us-east-2
[Finished ⬇️0.45% ⬆️0.0%] ursa-i9-9960x
[Finished ⬇️0.22% ⬆️0.04%] ursa-thinkcentre-m75q
Supported benchmarks:
ec2-t3-xlarge-us-east-2: Supported benchmark langs: Python. Runs only benchmarks with cloud = True
ursa-i9-9960x: Supported benchmark langs: Python, R, JavaScript
ursa-thinkcentre-m75q: Supported benchmark langs: C++, Java