[rubygems/rubygems] Set SOURCE_DATE_EPOCH env var if not provided.

Fixes #2290.

1. `Gem::Specification.date` returns SOURCE_DATE_EPOCH when defined,
2. this commit makes RubyGems set it _persistently_ when not provided.

This combination means that you can build a gem, check the build time,
and use that value to generate a new build -- and then verify they're
the same.

d830d53f59

lib/rubygems/package.rb

@@ -193,7 +193,7 @@ class Gem::Package
   def initialize(gem, security_policy) # :notnew:
     @gem = gem
 
-    @build_time      = ENV["SOURCE_DATE_EPOCH"] ? Time.at(ENV["SOURCE_DATE_EPOCH"].to_i).utc : Time.now
+    @build_time      = Gem.source_date_epoch
     @checksums       = {}
     @contents        = nil
     @digests         = Hash.new { |h, algorithm| h[algorithm] = {} }