[BUG] rapids_test_install_relocatable() does not play nice with gtest_discover_tests(DISCOVERY_MODE PRE_TEST)

[Jacobfaib]

Full description of the bug can be found here: https://gitlab.kitware.com/cmake/cmake/-/issues/26294 (specifically https://gitlab.kitware.com/cmake/cmake/-/issues/26294#note_1568638)...

...but the TL;DR is that the CTestInstall.cmake rewriting done by rapids_test_install_relocatable() eventually leads to include()-ing GoogleTestAddTests.cmake. This module -- if called in scripting mode (which it will be at install-time) -- will attempt to call a function which expects certain variables to be predefined. These variables are not defined, and hence install errors out.