ci: Generate Git tarballs on CI

In order to make Git repositories available in the shared download
cache, tarballs must be created. This feature is turned off by default
for performance reasons.

We are hitting upstream rate limits when fetching from Github
(particularly the Zephyr repositories), so by enabling this feature the
internal cache will be used instead of upstream.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>