When looking at the global Hudson config, there is a section for "environment
variables" with this help:
—
These key-value pairs apply for every build on every node. They can be used in
Hudson's configuration (as $key or ${key}) and will be added to the environment
for processes launched from the build.
—

Unfortunately, the Git plugin does not try to propagate the environment
variables to the invocation of the git executable. In my case, I want to put a
custom HOME location in so that SSH can properly locate the known hosts and keys
files for non-interactive login.