Revert "ci: Use bookworm image"

This reverts commit d7e27bc793.

Bookworm breaks build because it ships a newer version of rustlib-regex than supported.

.gitlab-ci.yml

@@ -1,4 +1,4 @@
-image: $CI_REGISTRY/world/phosh/phosh/debian:v0.0.20211208
+image: debian:bullseye
 
 stages:
   - build
@@ -6,7 +6,10 @@ stages:
 
 before_script:
   - apt-get -y update
-  - apt-get -y install ca-certificates
+  - apt-get -y install wget ca-certificates gnupg
+  - echo "deb [trusted=yes] http://ci.puri.sm/ bullseyeci main" > /etc/apt/sources.list.d/ci.list
+  - wget -O- https://ci.puri.sm/ci-repo.key | apt-key add -
+  - apt-get -y update
 
 build_docs:
   stage: build
@@ -53,7 +56,6 @@ build_deb:
     - cp ../*.deb .
 
 build_deb:arm64:
-  image: debian:bookworm
   tags:
     - aarch64
   stage: build