ci: codecov: Prioritise remote Redis cache storage

This commit updates the codecov workflow such that ccache only uses remote
Redis cache storage when available.

The purpose of this to reduce the individual runner local disk IOPS
requirement; thereby, reducing the overall load on the SAN.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>
(cherry picked from commit a636c52b)