ci: include more files in checksum generation

Created by: jhchabran

Follow-up to https://github.com/sourcegraph/sourcegraph/pull/38940

We had a quick sync with @valerybugakov and we agreed that it would be safer to:

• cover the missing files that weren't used to build up the index
• add a banner making it explicit that the bundle has been cached for the build

On his side, @valerybugakov will add in https://github.com/sourcegraph/sourcegraph/issues/39034 a way to check if the bundle came from the cache or not, so we could quickly see if that's the case or not.

Keep in mind that the cache we're seeing here, only applies to the server image, all others are not using the cache. This limits the blast radius of a possible issue with this optimization.

Test plan

Tested in CI again (cache miss + hit)