Always restore global environ if we use vfork

Always restore global environ if we use vfork

We may change the global environ variable in the child; when using
vfork() this also sets it in the parent.  Preserve the parent's
environ in child_execute_job() so it takes effect for all callers.

Reported by Denis Excoffier <bug-tar@Denis-Excoffier.org>
Root cause found by Martin Dorey <Martin.Dorey@hitachivantara.com>

* src/job.c (start_job_command): Remove save/restore of the parent
environment.
(child_execute_job): Add save/restore of the parent environment,
if we use vfork().
* tests/scripts/functions/shell: Add a test the crashes if we don't
reset environ after we run $(shell ...).