fix(ci): work around gitlab's variable substitution limitations

templates/nix-gitlab-ci.yml

@@ -4,8 +4,8 @@ spec:
       type: string
       description: |
         (empty for auto) | none | runner | cachix | attic
-        When left empty $NIX_CI_CACHE_STRATEGY will be used, which defaults to none
-      default: ""
+        Sets the default strategy and will be overridden by $NIX_CI_CACHE_STRATEGY
+      default: "none"
     cache_files:
       type: array
       description: |
@@ -19,17 +19,29 @@ stages:
 variables:
   # which version of the image should be used
   _NIX_CI_VERSION: ${NIX_CI_VERSION}
-  _NIX_CI_IMAGE: ${NIX_CI_IMAGE:-registry.gitlab.com/technofab/nix-gitlab-ci/nix-ci:${_NIX_CI_VERSION}}
+  _NIX_CI_IMAGE: registry.gitlab.com/technofab/nix-gitlab-ci/nix-ci:${_NIX_CI_VERSION}
   # force build the pipeline yaml
   _NIX_CI_FORCE_BUILD: ${NIX_CI_FORCE_BUILD}
   # disable caching on the child pipeline jobs
   _NIX_CI_DISABLE_CACHE: ${NIX_CI_DISABLE_CACHE}
   # type of cache strategy to use (none, runner, attic, cachix)
-  _CACHE_STRATEGY_TMP: $[[ inputs.cache_strategy ]]
-  _NIX_CI_CACHE_STRATEGY: ${NIX_CI_CACHE_STRATEGY:-${_CACHE_STRATEGY_TMP:-none}}
+  _NIX_CI_CACHE_STRATEGY: $[[ inputs.cache_strategy ]]
   # for multiple pipelines
-  _NIX_CI_PIPELINE_NAME: ${NIX_CI_PIPELINE_NAME:-${CI_PIPELINE_SOURCE:-default}}
+  _NIX_CI_PIPELINE_NAME: ${CI_PIPELINE_SOURCE}
+.env_overrides: &env_overrides
+  rules:
+    - if: $NIX_CI_IMAGE != null
+      variables:
+        _NIX_CI_IMAGE: $NIX_CI_IMAGE
+    - if: $NIX_CI_CACHE_STRATEGY != null
+      variables:
+        _NIX_CI_CACHE_STRATEGY: $NIX_CI_CACHE_STRATEGY
+    - if: $NIX_CI_PIPELINE_NAME != null
+      variables:
+        _NIX_CI_PIPELINE_NAME: $NIX_CI_PIPELINE_NAME
+    - when: on_success
 nix-ci:build:
+  <<: *env_overrides
   stage: build
   image: $_NIX_CI_IMAGE
   cache:
@@ -60,6 +72,7 @@ nix-ci:build:
     paths:
       - generated-gitlab-ci.yml
 nix-ci:trigger:
+  <<: *env_overrides
   stage: trigger
   needs:
     - nix-ci:build