borg2: avoid having the chunks index loaded multiple times

The chunks index can get quite big, thus we must avoid that it is multiple times in memory accidentally.

#10359 fixes this for `borg compact`.

`borg check` has the index loaded twice.

Start at the borg check entry point and compare its chunks-index loading path with the fix described in #10359 for borg compact. Verify where the index is loaded twice, then run borg check to confirm the index is loaded once and the command still completes normally.

Written by the indexing model from the issue text.