ci: Work around broken python UTF8 handling in the CI docker image

Gitlab very kindly exposes the details of the git commit message (among much else) in the environment. Unfortunately, piglit tries to handle the environment in non-UTF8-safe ways, which means if the top-of-tree commit mentions non-ASCII characters (say, in the author's name) then all the tests fail and so does the pipeline.

Fortunately none of those variables are things our piglit invocation needs. Since I've failed to rebuild the docker image as yet, just clear the likely variables from the environment before running piglit.

This-makes-me: ☹