[build] Fix unexpected warnings: No names found, cannot describe anyt…

…hing. (sonic-net#12963)

Why I did it
In PR check pipelines, there are too many duplicated warnings:
fatal: No names found, cannot describe anything.
SONIC_IMAGE_VERSION will not change in one build. We don't need to calculate in every reference. We just need calculate one time, then record it.
In Makefile, '=' will calculate again and again when it is referred.

How I did it
Fix it in Makefile.

How to verify it
Check this PR's check pipeline result.

slave.mk

@@ -11,7 +11,7 @@ UID = $(shell id -u)
 GUID = $(shell id -g)
 
 ifeq ($(SONIC_IMAGE_VERSION),)
-	override SONIC_IMAGE_VERSION = $(shell export BUILD_TIMESTAMP=$(BUILD_TIMESTAMP) && export BUILD_NUMBER=$(BUILD_NUMBER) && . functions.sh && sonic_get_version)
+	override SONIC_IMAGE_VERSION := $(shell export BUILD_TIMESTAMP=$(BUILD_TIMESTAMP) && export BUILD_NUMBER=$(BUILD_NUMBER) && . functions.sh && sonic_get_version)
 endif
 
 .SECONDEXPANSION: