bake-hcl: add function to access current time / build time

[tnaroska]

I was looking for a way to access the current time (or build time) in my docker-bake.hcl file but couldn't find an obvious way to do so.
There are some cty stdlib functions that work on timestamps (e.g. timeAdd, formatDate) but no obvious way to get the current time. I would suggest adding a wrapper function for time.Now() or maybe a builtin variable to access the build time.

Environment:

buildx   v0.8.2
buildkit v0.10.3