gitlab-ci: Set BUILD_ENV_OVERRIDE to customize the build

Long term reproducibility relies on being able to reproduce the build environment as closely as possible.

By setting the BUILD_ENV_OVERRIDE file variable in the GitLab CI/CD configuration when firing a pipeline it is possible to re-use the contents stored in the build-env.txt file from a previous build to reproduce it.

To do so:

• find the build-env.txt file from the artifacts of the build to be reproduced
• create a new branch in this repository pointing to the commit saved in the RECIPES_COMMIT field of build-env.txt
• go to Pipelines → Run Pipeline page on GitLab
• configure a variable of type File named BUILD_ENV_OVERRIDE
• paste the contents of build-env.txt there
• run the pipeline

For instance: https://gitlab.apertis.org/infrastructure/apertis-image-recipes/-/jobs/465767