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)