In evergreen.yml, set_task_expansion_macros is overriding expansions set earlier in bypass_compile_and_fetch_binaries.py

As we delegate more interaction with Evergreen into Python code, I suspect we will start to see more cases of multiple conflicting sources of truth around Evergreen configuration. As part of this ticket, or as part of followup work, we should converge on a single, standalone Python script to set expansions and define a clean, strict interface for a mapping of expansions to tasks/BVs/builds/projects. The interface and its rules should not live in evergreen.yml so they can be unit tested as part of the mongodb repo. (You can't, e.g. test code in the timeout section of an evergreen.yml in the same project using the same evergreen.yml)