add overridable `save-always` option

[IanButterworth]

Closes #79

[omus]

Have you validated this is working? This failed CI run didn't seem to save the cache:

• Failed job: https://github.com/julia-actions/cache/actions/runs/7572229335/job/20667038487?pr=107#step:11:123
• Successful job: https://github.com/julia-actions/cache/actions/runs/7572229335/job/20666823090?pr=107#step:13:211

[IanButterworth]

It appears to be a CI glitch. CI had stalled and I did a partial re-run only of the failed jobs, which might not be valid for the test setup here.

There's a macos runner stalling here now too

[IanButterworth]

I've been meaning to test this on another repo with forced failure. I'll check before merge

[IanButterworth]

Yeah this is broken (never worked) upstream. PR to fix it https://github.com/actions/cache/pull/1325

[IanButterworth]

There's a staggering lack of review urgency on the upstream fix PR https://github.com/actions/cache/pull/1325

It makes me wonder if we should maintain a fork of actions/cache

[omus]

Based upon this comment the change to actions/cache@v4 should be considered breaking here. I think we'll be merging #123 first which means this change will be non-breaking for the upcoming release (v2).

[IanButterworth]

actions/cache are planning to remove the (non-working) save-always option which I think makes the goal of this PR impossible.

https://github.com/actions/cache/issues/1315#issuecomment-2163822166

[DilumAluthge]

Marking as draft until (if/when) the blockers in https://github.com/actions/cache/issues/1315#issuecomment-2163822166 become unblocked.