Fixed lint warning

[cmaglie]

I did not test this PR, but it should write the DB without extra copying.

This solves the warning:

# arduino.cc/repository/internal/libraries/db
internal/libraries/db/db.go:237:34: call of json.MarshalIndent copies lock value: arduino.cc/repository/internal/libraries/db.DB contains sync.Mutex

[per1234]

@cmaglie would you mind rebasing the PR so we can get the DB integration test on it? That test will do everything I would normally do manually for reviewing this change, and likely do a better job than I would.

[cmaglie]

there you go! :-)