Add build_environment.yml for gitlab ci

This hopefully makes it easier to create new build runners.

.gitlab-ci.yml

@@ -8,6 +8,8 @@ basic_build_check:
     - build
   script:
     - source /var/lib/gitlab-runner/.bashrc
+    # create or update the existing environment to the newest pelican version
+    - conda env update -f build_environment.yml
     - conda activate rain_site
     - make clean
     - make publish