future_git_tag should override MONGO_VERSION

When we switched to precalculating MONGO_VERSION in SERVER-63191, we broke future_git_tag tests, which add a dummy tag to the repository so we can test what happens with an arbitrary future version. Unfortunately, the behavior of the build task depends on MONGO_VERSION not being precalculated, so it never gets overridden.

What we can do here is to simply generate and apply version expansions a second time for all of the future_git_tag tasks to ensure the version produced by the dummy git tag gets used.