buildfng: Moved the image and distro variables to global

The image and distro for the buildfng job can now be set via
global/external variables.

manifest.yml

@@ -19,6 +19,13 @@ variables:
   # Default image and distro
   CI_PARAM_IMAGE: guf-image
   CI_PARAM_DISTRO: guf-wayland
+  # Flash-N-Go image and distro
+  #
+  # In the past, the buildfng job overwrote the image and distro itself.
+  # Because of the transition to the new seconorth names, the image and
+  # distro for the buildfng must be settable from outside of the job.
+  CI_PARAM_IMAGE_FNG: fngsystem-image
+  CI_PARAM_DISTRO_FNG: guf-fngsystem
 
 workflow:
   rules:
@@ -126,8 +133,8 @@ changelog:
   extends:
     - .buildimage
   variables:
-    CI_PARAM_IMAGE: fngsystem-image
-    CI_PARAM_DISTRO: guf-fngsystem
+    CI_PARAM_IMAGE: ${CI_PARAM_IMAGE_FNG}
+    CI_PARAM_DISTRO: ${CI_PARAM_DISTRO_FNG}
 
 build:merge_request:
   extends: .infrastructure