A minor bauble of a Robert Altman film based on Garrison Keillor’s fabled public-radio variety show. It’s like the Grand Ole Opry in small-town miniature — but, in its folksy chamber-music way, it’s a delight. What seals it is the performers’ belief in their shaggy-dog selves. With Meryl Streep, Woody Harrelson, and Lindsay Lohan.