Respect SOURCE_DATE_EPOCH environment variable (if set) over Time.now

[lamby]

Whilst working on the Reproducible Builds effort [0], we noticed
that jekyll builds sites that are not reproducible.

This is because site.time uses the current time of day and thus
anything then shipping a Jekyll site (eg. as documentation) will
therefore not be reproducible.

This commit prefers the value in the SOURCE_DATE_EPOCH environment
variable [1] if it is set and config.time is also not set.

[0] https://reproducible-builds.org/
[1] https://reproducible-builds.org/specs/source-date-epoch/

[ghost]

@lamby Can you fix the RuboCop offense as well as add a simple test? Thanks so much!

[ashmaroli]

@DirtyF For a given Jekyll::Site instance site, calling site.now should not set site.time. Only site.reset should do that..
So can I request you to amend your commit to simply do the following:

def reset
  self.time = now
  self.layouts = {}
  self.pages = []
  [...]
end

# preferably a private method
def now
  # your implementation
end