ClusterFuzz runs of the `fuzz_submodule` target have been failing
because the `git` import was placed before the condition that sets the
Git executable path.

The order in which `git` is imported matters because it attempts to find
a Git executable as the import is loaded (via `refresh()` in
`git/__init__.py`.) As per gitpython-developers#1909, we configure the ClusterFuzz
environment to use a bundled Git executable via the env variable
condition in all fuzz targets.

approved these changes Jun 4, 2024