releng: Update to Tycho 0.26.0

releng: Update to Tycho 0.26.0

Also skip archiving the repository.
This new option in Tycho 0.26.0 saves a few seconds of build time
since we do not use the archive for either p2 repositories
(the releng-site and the rcp one).

Change-Id: Iec47d66dda72e9b686ed9d5e8f2c54ffd0983254
Signed-off-by: Marc-Andre Laperle <marc-andre.laperle@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/78974
Reviewed-by: Hudson CI
Reviewed-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Tested-by: Bernd Hufmann <bernd.hufmann@ericsson.com>