Drop the APT GPG key from images that get it via the base container

The base container already ships the GPG key for the Apertis APT
repositories, no need to have multiple copies of it.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>

image-builder/test-recipe.yaml

@@ -12,5 +12,5 @@ actions:
     mirror: {{ $mirror }}
     variant: minbase
     keyring-package: apertis-archive-keyring
-    keyring-file: keyring/apertis-archive-keyring.gpg
+    keyring-file: /etc/apt/trusted.gpg.d/apertis-archive-keyring.gpg
     merged-usr: true