Make cache key timestamping optional.

[fruffy]

Prompted by #117.

For Github repositories with many different actions and concurrent PRs, ccache-action's timestamps can quickly fill up the cache and evict other cached entries. This PR adds an option to disable timestamping.

I am not familiar with either Javascript or Github Actions, so suggestions on how to improve this are very much welcome.

[ppizarror]

We have the same problem! Having this option might help us a lot @hendrikmuhs

[hendrikmuhs]

Looks good to me, instead of useTimestamp I suggest appendTimestamp.

[hendrikmuhs]

Thanks! I can merge and fix the js files myself. The last dependabot updates broke the build anyway.

[ppizarror]

Nice!! thanks a lot @hendrikmuhs, @fruffy

[vadz]

I was eager to try this as I also don't like having one cache for each of the runs, but it seems that this doesn't actually work, as saving the cache always fails when using the new option and the cache already exists.

According to this comment, an existing cache can't be overwritten, and if this is correct, then the new option is not very useful. Or am I missing something here?

[fruffy]

Part of the motivation for this PR was to get some control over the key signature. You can still define a key that is flexible enough to avoid that issue (using PR/commit IDs). This is what we have been doing.

But yes, it is an issue right now. Hope they will fix that bug at some point.

[vadz]

Thanks for the clarification, it wasn't obvious that using this option required making the key unique in some other way. I think it would be nice to mention this in the README.