support/download: drop sub-second precision in tarball creation

support/download: drop sub-second precision in tarball creation

Some download backends, like svn, will provide timestamps with a
sub-second precision, e.g.

    $ svn info --show-item last-changed-date [...]
    2021-02-19T20:22:34.889717Z

However, the PAX headers do not accept sub-second precision, leading to
failure to download from subversion:

    tar: Time stamp is out of allowed range
    tar: Exiting with failure status due to previous errors
    make[1]: *** [package/pkg-generic.mk:148: [...]/build/subversion-1886712/.stamp_downloaded] Error 1

Fix that by massaging the timestamp to drop the sub-second part. We
do that in the generic helper, rather than the svn backend, so that
all callers to the generic helper benefit from this, as this is more
an internal details of the tarball limitations, than of the backends
themselves.

Reported-by: Roosen Henri <Henri.Roosen@ginzinger.com>
Signed-off-by: Vincent Fazio <vfazio@xes-inc.com>
[yann.morin.1998@free.fr:
  - add Henri as reporter
  - move it out of the svn backend, and to the generic helper
  - reword the commit log accordingly
  - use an explicit time format rather than -Iseconds
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>