add initial support for SOURCE_DATE_EPOCH

[DannyBrito]

What this PR does / why we need it:
Adds initial support for SOURCE_DATE_EPOCH, technically this variable was already able to be injected in Dalec specs as one of the requirements to add this is to explicit require the args/envs. It seems like SOURCE_DATE_EPOCH env var is auto injected by docker/buildx so no need to explicit --build-arg. The difference I could notice is that now with change applied the build will not fail as unknow arg for SOURCE_DATE_EPOCH var but it still requires the spec to set this variable to be used.

# syntax=ghcr.io/azure/dalec/frontend:latest
args:
  SOURCE_DATE_EPOCH: # [Required] Needs to explicit call for variable to be usable within spec

name: go-md2man
version: 2.0.3
packager: Dalec Example
...
sources:
...
build:
  env:
    SOURCE_DATE_EPOCH: ${SOURCE_DATE_EPOCH} # [Required] Needs to explicit set variable to be usable within build scope
  steps:
    - command: |
        set -x
        echo "${SOURCE_DATE_EPOCH} # If any of the required fields is missing value will be empty.
        exit 1

Which issue(s) this PR fixes (optional, using fixes #<issue number>(, fixes #<issue_number>, ...) format, will close the issue(s) when the PR gets merged):
Fixes #54

Special notes for your reviewer:

[cpuguy83]

LGTM

[AkihiroSuda]

[Required] Needs to explicit call

WDYT about defining SOURCE_DATE_EPOCH by default?
The docker/dockerfile frontend doesn't do that for compatibility with existing Dockerfiles though.

[cpuguy83]

@AkihiroSuda I personally prefer the explicit declaration.

[cpuguy83]

After giving this some more thought, I think it would be good to not require SOURCE_DATE_EPOCH to be explicitly set in the spec.

[cpuguy83]

To clarify my reasoning here, if someone wants to build a spec, they shouldn't have to modify it to get the benefits of SOURCE_DATE_EPOCH in the build itself.

[cpuguy83]

Opened #447