Downstream configure fails when defining local `CELERITAS_USE_X`

[sethrj]

We export many CELERITAS_ variables to our installed CeleritasConfig.cmake file; many of them (e.g. CELERITAS_USE_X) found by introspecting the cache variable properties. However, in SCALE (and this is legal in general) we locally set CELERITAS_USE_VecGeom to OFF, unless the user has a default. This means that Celeritas' cmake sees it as set, so it works during the build, but it doesn't export it to the CeleritasConfig.cmake. For the vecgeom case, this breaks the include of CeleritasLibrary.cmake. Workaround is to set variables upstream as cache.