core/reproducible: do not override SOURCE_DATE_EPOCH

core/reproducible: do not override SOURCE_DATE_EPOCH

SOURCE_DATE_EPOCH is currently forcibly set (to either the git commit
date, or the last release date).

However, the spec mandates that it should not be modified if already
set: https://reproducible-builds.org/specs/source-date-epoch/

    Build systems MUST NOT overwrite this variable for child
    processes to consume if it is already present.

Abide by the rule, and only set it if not already set.

This will allow users to pass it from an upper-layer buildsystem (e.g. a
jenkins or gitlab-ci job, for example), when they have a reson to do so.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Reported-by: Einar Jón Gunnarsson <tolvupostur@gmail.com>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Einar Jón Gunnarsson <tolvupostur@gmail.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>