Log file - add full timestamp

[kapitainsky]

At the moment when using log-file option only time of events is recorder:

13:35:36 [INFO] using config ./rustic.toml
13:35:38 [INFO] repository opendal:test-dal-lock: password is correct.
13:35:38 [INFO] using cache at /Users/kptsky/Library/Caches/rustic/79622816e3c9c922340f1fd7b7fd31f225f7a1d14faaeea5d6d52386980d16ce
13:35:38 [INFO] using all backup sources from config file.
13:35:38 [INFO] merging source="/private/tmp/rsnap-79622816" section from config file
13:35:39 [INFO] using parent 126c1ce1
13:35:39 [INFO] starting to backup "/private/tmp/rsnap-79622816"...
13:35:39 [INFO] backup of "/private/tmp/rsnap-79622816" done.
13:36:12 [INFO] using config ./rustic.toml

It would be much better if full timestamp is used instead, including date, e.g.:

Thu Feb 22 13:41:15 GMT 2024 [INFO] using config ./rustic.toml
Thu Feb 22 13:41:17 GMT 2024 [INFO] repository opendal:test-dal-lock: password is correct.