warn users that `lfs: true` can bloat bandwidth consumption

[maxheld83]

We learned recently that setting lfs: true can really bloat the bandwidth consumption of git lfs data packs, because this setting will make the checkout action download all pointers on every commit (apparently?).

It would be great if either:

1. this wouldn't count towards the quota, since this is presumably, all inside gh datacenter
2. there would be a transparent caching layer behind it to avoid this (as per #165)
3. barring 1 and 2, the docs should warn users that this will happen.

[klausbadelt]

Agree with all of the above. As temporary workaround we're using nschloe/action-cached-lfs-checkout currently.

[kelle]

Yes, user should be warned but even better would be adding a cache: True option to the next version of checkout.

In the meantime, the implementation provided by @nschloe worked for me: https://github.com/actions/checkout/issues/165#issuecomment-1639209867