Add name output to identify builder instance name

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

setup-buildx/action.yml

@@ -27,6 +27,8 @@ inputs:
     required: false
 
 outputs:
+  name:
+    description: 'Builder instance name'
   platforms:
     description: 'Available platforms (comma separated)'