Don't install vendored backward files

[scpeters]

🦟 Bug fix

Follow-up to #2776

Summary

We vendored backward-cpp in #2776 to use in integration tests, but I noticed while investigating an unstable nightly build that some vendored files are being installed with gz-sim. I don't think these should be installed with our packages, so I am modifying the cmake logic to prevent their installation here.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[j-rivero]

The patch works although it is fragile if at some point we update the version of the backward vendor copy. Since the effects of installing the files are not severe I'm inclined to to merge the change.

An alternative would be to deal with file removal in packaging but the problem is not solve for source compilations and we scale the size of the problem to many packaging platforms.