Fix #8522: Add {unixtime} placeholders (1.4-maint) (#8675)

src/borg/helpers/parseformat.py

@@ -218,6 +218,7 @@ def replace_placeholders(text, overrides={}):
         'hostname': hostname,
         'now': DatetimeWrapper(current_time.astimezone(None)),
         'utcnow': DatetimeWrapper(current_time),
+        'unixtime': int(current_time.timestamp()),
         'user': getosusername(),
         'uuid4': str(uuid.uuid4()),
         'borgversion': borg_version,