Add a cron job to touch ccache entries to keep GH from dropping them after 7 days

[bleggett]

We use ccache with github save/restore to dramatically cut kernel build times.

This works well, but GH has a 10GB limit for all cache entries, and a 7-day TTL for each cache entry.

Which means that if we don't build a kernel for a week,we lose our ccache benefit entirely, which stinks.

The correct way to work around this is to replace GH's cache action with one that saves/restores directly from a dedicated S3 bucket we set up and manage, such as sccache, so we can manage storage and TTLs ourselves.

What this PR does in the meantime is autorun an action that saves/restores the cache every 4 days, well within the 7-day TTL, to keep GH from expiring them. Which is disgusting, but cheap/simple.

xref: #139

[bleggett]

Build was cancelled because of runner contention, but CI won't touch or validate this change anyway.