bake: allow annotations to be set on the command line

Annotations were not merged correctly. The overrides in `ArrValue` would
be merged, but the section of code setting them from the command line
did not include `annotations` in the list of available attributes so the
command line option was completely discarded.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

docs/reference/buildx_bake.md

@@ -354,6 +354,7 @@ $ docker buildx bake --set foo*.no-cache              # bypass caching only for
 
 You can override the following fields:
 
+* `annotations`
 * `args`
 * `cache-from`
 * `cache-to`