SCORU/Node: fix permissions for store data file when run in readonly

Context

When loaded in readonly mode, an indexed file base store would create its data file with incorrect permissions. This made it impossible to load a store in read-write mode that was first loaded in readonly mode. This MR fixes this issue.

Manually testing the MR

dune exec src/lib_layer2_store/test/main.exe  -- --file test_indexed_store.ml -t 'tezos-layer2-store: indexed-store-unit (load)'