make: also preserve PYTHON_EXE across UNSET_AND_MAKE

PYTHON_EXE follows the same `?=` then `export := $(PYTHON_EXE)` pattern
as OPENROAD_EXE / OPENSTA_EXE / YOSYS_EXE, so a caller-supplied value is
similarly stripped by UNSET_AND_MAKE because the re-export gives it
origin "file". Add PYTHON% to the exclusion list alongside the other
tool prefixes.

Addresses gemini-code-assist review feedback on PR #4159.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>

Author: oharboe

flow/scripts/variables.mk

@@ -188,7 +188,7 @@ export RESULTS_V = $(notdir $(sort $(wildcard $(RESULTS_DIR)/*.v)))
 export GDS_MERGED_FILE = $(RESULTS_DIR)/6_1_merged.$(STREAM_SYSTEM_EXT)
 
 define get_variables
-$(foreach V, $(.VARIABLES),$(if $(filter-out $(1), $(origin $V)), $(if $(filter-out .% %QT_QPA_PLATFORM% KLAYOUT% OPENROAD% OPENSTA% YOSYS% GENERATE_ABSTRACT_RULE% do-step% do-copy% OPEN_GUI% OPEN_GUI_SHORTCUT% SUB_MAKE% UNSET_VARS% export%, $(V)), $V$ )))
+$(foreach V, $(.VARIABLES),$(if $(filter-out $(1), $(origin $V)), $(if $(filter-out .% %QT_QPA_PLATFORM% KLAYOUT% OPENROAD% OPENSTA% PYTHON% YOSYS% GENERATE_ABSTRACT_RULE% do-step% do-copy% OPEN_GUI% OPEN_GUI_SHORTCUT% SUB_MAKE% UNSET_VARS% export%, $(V)), $V$ )))
 endef
 
 export UNSET_VARIABLES_NAMES := $(call get_variables,command% line environment% default automatic)