build: specify platforms explicitly (to workaround regression in Turbo Bob)

Author: joonas-fi

.config/turbobob.json

@@ -5,9 +5,8 @@
 	"docker_images": [
 		{
 			"image": "fn61/buildkit-golang",
-			"dockerfile_path": "Dockerfile",
 			"tag_latest": true,
-			"auth_type": "creds_from_env"
+			"platforms": ["linux/amd64"]
 		}
 	]
 }