Fix handling TEMPLATE_LABEL when calculating template name

Yet another case of make not passing exported variables to $(shell ...)
(https://savannah.gnu.org/bugs/?10593). Here it applies when
builder.conf was loaded in the same make instance - make
--eval='include builder.conf' call from check-release-status script.

QubesOS/qubes-issues#3935

Makefile

@@ -23,7 +23,9 @@ TEMPLATE_ENV_WHITELIST += \
 	CENTOS_MIRROR EPEL_MIRROR QUBES_MIRROR
 
 # Make sure names are < 32 characters, process aliases
-fix_up := $(shell TEMPLATE_NAME=$(TEMPLATE_NAME) ./builder_fix_filenames)
+fix_up := $(shell TEMPLATE_NAME=$(TEMPLATE_NAME) \
+		TEMPLATE_LABEL="$(TEMPLATE_LABEL)" \
+		./builder_fix_filenames)
 TEMPLATE_NAME := $(word 1,$(fix_up))
 
 export TEMPLATE_NAME