[cmake] Force-set the ZLIB_LIBRARY

Apparently cmake sets the cache after the configuration is complete and using
FORCE updates the cache including the relevant generator expressions.

This fixes the case where we build ROOT with -Dbuiltin_zlib=Off and then LLVM
sanity checks the found zlib issuing:

```
Error evaluating generator expression:

  $<TARGET_FILE:ZLIB>

No target "ZLIB"
```

builtins/zlib/CMakeLists.txt

@@ -126,7 +126,7 @@ endif()
 
 add_library(ZLIB::ZLIB ALIAS ZLIB)
 
-set(ZLIB_LIBRARY $<TARGET_FILE:ZLIB> CACHE INTERNAL "")
+set(ZLIB_LIBRARY $<TARGET_FILE:ZLIB::ZLIB> CACHE INTERNAL FORCE "")
 set(ZLIB_LIBRARIES ZLIB::ZLIB CACHE INTERNAL "")
 
 if(DEFINED ZLIB_LIBRARY_DEBUG)