Meta: Don't install CA certificates during a on-Serenity build

`SERENITYOS` is also set when compiling Lagom on SerenityOS, so we can't
just check it and expect `CMAKE_STAGING_PREFIX` to be set.

Instead, check `CMAKE_STAGING_PREFIX` directly and use that as an
indicator for whether we can install a file there.

Meta/CMake/ca_certificates_data.cmake

@@ -14,7 +14,7 @@ if (ENABLE_CACERT_DOWNLOAD)
 
     download_file("${CACERT_URL}" "${CACERT_PATH}/${CACERT_FILE}")
 
-    if (SERENITYOS)
+    if (NOT "${CMAKE_STAGING_PREFIX}" STREQUAL "")
         set(CACERT_INSTALL_PATH ${CMAKE_STAGING_PREFIX}/etc/${CACERT_INSTALL_FILE})
     else()
         set(CACERT_INSTALL_PATH ${CMAKE_CURRENT_BINARY_DIR}/${CACERT_INSTALL_FILE})