Backport !4029 - Proto/Baker: load cache lazily

Context

Changes in !4029 (merged) touch src/proto_alpha and we need them in snapshotted Protocol I. Cherry-picked commit from it using the following commands.

$ git checkout -b proto-i-backport-4029
$ git cherry-pick 07cf02c7aa6f8af05091e0cfdd09e96b5335da66

Manually testing the MR

Check CI.