ci: update the message for unavailble third-party software

ci: update the message for unavailble third-party software

An earlier fix added an extra message immediately after failing to
download a third-party package.  But near the end of the script,
their availability is checked again and given a message.

Remove the new ones added with a recent fix, as they are redundant.
If we were to add more places to download these software (e.g. for
other platforms we currently do not download them on), the existing
warnning near the end of the script will also trigger.

While at it, as Dscho suggests, rewrite the WARNING: label on the
warning message to ::warning::, which presumably should be shown a
bit more prominently in the CI summary.

Suggested-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>