Move validate step into Nix script

.gitlab-ci.yml

@@ -34,6 +34,8 @@ distros:
 
 validate:
   stage: lint
+  tags:
+    - nix
   needs:
     - generate
   cache:
@@ -42,12 +44,7 @@ validate:
           - sources/build.pkr.hcl
       paths:
         - ./packer_config/
-  script: |-
-    set -exo pipefail
-    packer init sources
-    for vars in $(find distros/ -name '*.pkrvars.hcl'); do
-      packer validate -except=upload -var-file=${vars} sources/
-    done
+  script: nix run ".#validate"
 
 shellcheck:
   stage: lint