feat: add variable `commit_date` #6952

Job	Run time
custom-labels-annotations	10s
context (workflow)	4s
images	11s
context (git)	4s
flavor	14s
multi-images	10s
tag-schedule	5s
tag-semver (auto)	6s
tag-schedule (cron-{{date 'YYYYMMDD'}})	9s
tag-semver (true)	11s
tag-schedule ({{date 'YYYYMMDD-HHmmss'}})	8s
tag-semver (false)	5s
tag-schedule (schedule)	7s
bake	16s
bake-annotations	32s
bake-cwd	19s
docker-push	32s
global-exps	6s
json	7s
no-images	11s
output-env	20s
sep-tags ( )	17s
sep-tags (,)	8s
dump	17s
sha-short	6s
sha-short (16)	13s
tag-match (\d.\d.\d, 0)	7s
tag-match (\d.\d, 0)	7s
tag-match (v(.*), 1)	10s
	5m 32s

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