Preflight test does not have the same environment variables as wpull inside the pipeline

[JustAnotherArchivist]

Cf. #490 and #491

Environment variables on the preflight test are not modified, but inside the pipeline, only the selected ones specified in wpull_env in pipeline.py are passed to wpull. This means that the preflight test might succeed but the real wpull process might hang, as in that scenario of a small separate /tmp partition with TMPDIR set to something else to keep wpull from putting anything there or monitoring its disk space.