Previously the CI variable instance cache always attempted to read
Redis for a timestamp to determine whether the value was
up-to-date. The problem was that in a system with multiple processes
reading from the cache, the timestamp would constantly be advancing,
making it appear as though the cache always were stale. Not only did
this incur one Redis call per lookup, but also this would result in
frequent cache misses.

Since we mostly just use this cache data set in the creation of
pipelines, we can likely live with a 30-second delay between the time
when the variable is set and when it is live.

For the `gitlab-org/gitlab` project, this should reduce the overall
pipeline creation time by around 1.5 seconds (11% of the time), not to
mention reduce the network and memory overhead.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/322386

Changelog: fixed