CI: Set image to pureos/byzantium

Previously we relied on the default image set for the runner,
which technically can be anything and isn't even guaranteed
to include apt/dpkg.

.gitlab-ci.yml

@@ -18,6 +18,7 @@ before_script:
 
 build:aarch64:
   <<: *tags
+  image: pureos/byzantium
   stage: build
   allow_failure: true
   variables:
@@ -34,6 +35,7 @@ build:aarch64:
     expire_in: 1 week
 
 build:deb:
+  image: pureos/byzantium
   tags:
     - librem5:arm64
   stage: build