buildctl: consider propagating SOURCE_DATE_EPOCH from client env to build arg by default

[AkihiroSuda]

buildkit/docs/build-repro.md

Lines 50 to 51 in c7e7f44

	The `buildctl` CLI does not automatically propagate the `$SOURCE_DATE_EPOCH` environment value from the client host to the `SOURCE_DATE_EPOCH` build arg.
	However, higher level build tools, such as Docker Buildx (>= 0.10), may automatically capture the environment value.

I guess we can safely propagate SOURCE_DATE_EPOCH from client env to build arg by default. I don't remember why we didn't do so in the first place though.

Related:

• Propagate SOURCE_DATE_EPOCH from the client env docker/buildx#1482