Fix offset generation cache

[MrAlias]

I noticed that if I rebuild the offsetgen tool, delete the entries from the json file, and run it for the first time it gets it right, and the second time it puts all these wrong values, so I'm guessing there is some bug in the cache mechanism there.

Originally posted by @RonFed in #740 (comment)

[damemi]

+1 I'm also seeing weird behavior with the cache when trying to add fields in #790. Just running make docker-offsets caused the existing versions to get wiped and replaced with repeated {} entries. Deleting .tools/ and internal/pkg/inject/offset_results.json then re-running the tool seems to work

[MrAlias]

@damemi can you check if #781 resolves your issue?