test: ltp: disable content trust

Signing is not setup for this repositories. It seems it hasn't been updated
since before trust was added since the previous Makefile metadata (which was
translated in build.yml) did not contain `NOTRUST=1`.

Signed-off-by: Ian Campbell <ijc@docker.com>

test/pkg/ltp/build.yml

@@ -1,4 +1,5 @@
 image: test-ltp
 network: true
+disable-content-trust: true
 arches:
   - amd64