Export PROXYSQLGENAI to support building all packages with GenAI

When running `PROXYSQLGENAI=1 make packages`, the flag needs to be
passed to all sub-make processes spawned for each distribution.
Adding `export PROXYSQLGENAI` ensures that sub-makes inherit the
flag, so all packages are built with GenAI support and the correct
version number (e.g., 4.0.6 instead of 3.0.6).

This allows building all packages with GenAI using:
  PROXYSQLGENAI=1 make packages

Individual package builds still work:
  PROXYSQLGENAI=1 make ubuntu24
  PROXYSQLGENAI=1 make debian12-dbg
  etc.

Makefile

@@ -35,6 +35,7 @@ ifeq ($(CURVER_CHECK),0)
 endif
 
 export CURVER
+export PROXYSQLGENAI
 
 ### NOTES:
 ### SOURCE_DATE_EPOCH is used for reproducible builds