Recently we improved our evergreen.yml environment variable management by introducing the test_env_vars and configure_env_vars variables, making it explicit what environment we want for each buildvariant. However this requires that we prepend `${test_env_vars|}` to any calls in  shell commands that depend on the mongotoolchain and it can be easy to miss this prefix. We discovered a few uncovered cases when the non-mongotoolchain version of python was upgraded on ubuntu hosts and caused a handful of test failures.

We should investigate if we can instead set these environment variables at the start of each script. Ideally with an automated process that ensures we're always using a consistent mongotoolchain.