feat: add variable `commit_date` #6939

# to view logs

Job	Run time
flavor	9s
multi-images	13s
custom-labels-annotations	10s
images	11s
tag-match (\d.\d.\d, 0)	7s
tag-schedule	5s
tag-match (\d.\d, 0)	10s
tag-semver (auto)	12s
tag-schedule (cron-{{date 'YYYYMMDD'}})	7s
tag-semver (true)	7s
tag-schedule ({{date 'YYYYMMDD-HHmmss'}})	8s
tag-match (v(.*), 1)	7s
tag-semver (false)	8s
tag-schedule (schedule)	15s
bake	25s
bake-annotations	22s
bake-cwd	17s
docker-push	32s
global-exps	9s
json	6s
no-images	6s
output-env	30s
sha-short	7s
sep-tags ( )	6s
sha-short (16)	10s
sep-tags (,)	5s
	5m 4s

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