internal/repos: Wrap error from Store.Transact for context

Created by: indradhanush

We're seeing frequent "starting transaction: context deadline exceeded" errors on production. Wrapping the errors to add more context could help narrowing down the cause.

Link to logs: https://sourcegraph.grafana.net/explore?orgId=1&left=%5B%22now-6h%22,%22now%22,%22grafanacloud-sourcegraph-logs%22,%7B%22expr%22:%22%7Bapp%3D%5C%22repo-updater%5C%22,%20namespace%3D%5C%22prod%5C%22%7D%20%7C%3D%20%5C%22context%20deadline%20exceeded%5C%22%22%7D%5D