CMake: Fix install not finding external arrow for dynamic linking

[traversaro]

Before this PR, if rerun_sdk was build with RERUN_DOWNLOAD_AND_BUILD_ARROW set to OFF and RERUN_ARROW_LINK_SHARED set to ON, the downstream build of a package that uses rerun_sdk via find_package(rerun_sdk) had the following problems:

• A "Rerun got built with an automatically downloaded version of libArrow" warning was printed, even if rerun was built against a system version of arrow
• If the arrow system installation did not shipped Arrow::arrow_static (as it is the case for conda-forge, at least for the libarrow package), the CMake failed as the non-existent Arrow::arrow_static target was referenced.

These two problems are solved in these PR by:

• Only printing the "Rerun got built with an automatically downloaded version of libArrow" warning if @RERUN_DOWNLOAD_AND_BUILD_ARROW@ is OFF
• Define the rerun_arrow_target target using the value of RERUN_ARROW_LINK_SHARED used when building rerun.

What

Checklist

• I have read and agree to Contributor Guide and the Code of Conduct
• I've included a screenshot or gif (if applicable)
• I have tested the web demo (if applicable):
  • Using newly built examples: app.rerun.io
  • Using examples from latest main build: app.rerun.io
  • Using full set of examples from nightly build: app.rerun.io
• The PR title and labels are set such as to maximize their usefulness for the next release's CHANGELOG
• If applicable, add a new check to the release checklist!

• PR Build Summary
• Docs preview
• Examples preview
• Recent benchmark results
• Wasm size tracking

[Wumpf]

great catch, thanks again!

[traversaro]

great catch, thanks again!

Thanks for the fast feedback!