Use var/browser_release_date in tools/signing/gatekeeper-bundling.sh
As part of the signing process we need to re-generate a dmg file after
signing its content, which is done in
tools/signing/gatekeeper-bundling.sh (and ddmg.sh). In ddmg.sh
we're setting the modification date to 2000-01-01 01:01:01. Instead we
could use the release date from rbm.conf (defined as
var/browser_release_date).
We're already doing that for the unsigned dmg files generate during the
build, but the modification date gets reset to 2000-01-01 while
signing.