repo last_modified: use iso8601 timestamp format with --json

like yyyy-mm-ddThh:mm:ss - no tz yet, this likely needs more refactoring
to tz aware and utc datetime objects everywhere, currently there are
naive datetime objects and also localtime at quite some places.

(cherry picked from commit 32174dd9c885e2267a939cb81fe2d4130308d487)

src/borg/helpers.py

@@ -2212,7 +2212,7 @@ def basic_json_data(manifest, *, cache=None, extra=None):
             'mode': key.ARG_NAME,
         },
     })
-    data['repository']['last_modified'] = format_time(to_localtime(manifest.last_timestamp.replace(tzinfo=timezone.utc)))
+    data['repository']['last_modified'] = isoformat_time(to_localtime(manifest.last_timestamp.replace(tzinfo=timezone.utc)))
     if key.NAME.startswith('key file'):
         data['encryption']['keyfile'] = key.find_key()
     if cache: