skip jobs that don't make artifacts on main branch

This is okay because we only allow fast-forward merges, changes always
go through MRs, and MRs are subject to, e.g., the code quality checks.
This change is desirable because it should save some time and energy.

.gitlab-ci.yml

@@ -35,11 +35,18 @@ before_script:
   # Set CARGO_HOME to a cacheable path
   - export CARGO_HOME="$(git rev-parse --show-toplevel)/.gitlab-ci.d/cargo"
 
-ci:
+
+cache-ci-deps:
   stage: ci
   image: nixos/nix:2.18.2
   rules:
     - if: $CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH
+  script: direnv exec . job cache-ci-deps
+
+ci:
+  stage: ci
+  image: nixos/nix:2.18.2
+  rules:
     - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
   script:
     - direnv exec . job ci

bin/job

@@ -21,6 +21,12 @@ job_pages() (
     chmod u+w -R public
 )
 
+job_cache-ci-deps() (
+    set -euo pipefail
+
+    nix-build-and-cache ci
+)
+
 bail() (
     set -euo pipefail