[fix] version format string generated by 'git show'

Newer versions of git [1] do no longer support a format string that includes a minus
to remove leading zeros [2].  The format string '%Y.%m.%d'  is more version rod.

[1] https://github.com/searxng/searxng/issues/2413#issuecomment-1542320387
[2] https://github.com/searxng/searxng/pull/2122/files

Closes: https://github.com/searxng/searxng/issues/2413


Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>