Pass JULIA_NUM_THREADS=1 to the sysimage builder (#56695)

Having multiple threads isn't suported during the build since the
scheduler doesn't exist at this point. Fixes
#56533

sysimage.mk

@@ -65,14 +65,14 @@ RELDATADIR := $(call rel_path,$(JULIAHOME)/base,$(build_datarootdir))/ # <-- mak
 
 $(build_private_libdir)/basecompiler.ji: $(COMPILER_SRCS)
 	@$(call PRINT_JULIA, cd $(JULIAHOME)/base && \
-	$(call spawn,$(JULIA_EXECUTABLE)) -C "$(JULIA_CPU_TARGET)" $(HEAPLIM) --output-ji $(call cygpath_w,$@).tmp \
+	JULIA_NUM_THREADS=1 $(call spawn,$(JULIA_EXECUTABLE)) -C "$(JULIA_CPU_TARGET)" $(HEAPLIM) --output-ji $(call cygpath_w,$@).tmp \
 		--startup-file=no --warn-overwrite=yes -g$(BOOTSTRAP_DEBUG_LEVEL) -O1 Base_compiler.jl --buildroot $(RELBUILDROOT) --dataroot $(RELDATADIR))
 	@mv $@.tmp $@
 
 $(build_private_libdir)/sys.ji: $(build_private_libdir)/basecompiler.ji $(JULIAHOME)/VERSION $(BASE_SRCS) $(STDLIB_SRCS)
 	@$(call PRINT_JULIA, cd $(JULIAHOME)/base && \
 	if ! JULIA_BINDIR=$(call cygpath_w,$(build_bindir)) WINEPATH="$(call cygpath_w,$(build_bindir));$$WINEPATH" \
-			$(call spawn, $(JULIA_EXECUTABLE)) -g1 -O1 -C "$(JULIA_CPU_TARGET)" $(HEAPLIM) --output-ji $(call cygpath_w,$@).tmp $(JULIA_SYSIMG_BUILD_FLAGS) \
+			JULIA_NUM_THREADS=1 $(call spawn, $(JULIA_EXECUTABLE)) -g1 -O1 -C "$(JULIA_CPU_TARGET)" $(HEAPLIM) --output-ji $(call cygpath_w,$@).tmp $(JULIA_SYSIMG_BUILD_FLAGS) \
 			--startup-file=no --warn-overwrite=yes --sysimage $(call cygpath_w,$<) sysimg.jl --buildroot $(RELBUILDROOT) --dataroot $(RELDATADIR); then \
 		echo '*** This error might be fixed by running `make clean`. If the error persists$(COMMA) try `make cleanall`. ***'; \
 		false; \