sysroot: create sysroot metadata and publish it

Fix bogus path for sysroot tarball and generate metadata long it.

Signed-off-by: Héctor Orón Martínez <hector.oron@collabora.co.uk>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Differential Revision: https://phabricator.apertis.org/D7373