[osgearth] Fix naming of target and exported variables

[cbrl]

Describe the pull request Fixes some errors with the osgEarth CMake config introduced in #26024.

• What does your PR fix?

  The change to the target name suggested in https://github.com/microsoft/vcpkg/pull/26024#discussion_r933693922 only covered that single usage of the target. It needs to be followed up by changing the exported variables and the call to set_target_properties to match the new target name.

• Which triplets are supported/not supported? Have you updated the CI baseline?

  All, No

• Does your PR follow the maintainer guide?

  Yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  Yes