Fix tests in test_git with hard coded env setter
Fix tests in test_git with hard coded env setter

Commands for setting environment variables are platform specific. Tests are failing on Windows, as
they have harded coded commands for setting environment variables. This commit changes to use the
`get_envrionment_variable_setter_command` method in process_utils module to make the tests
cross-platform.

box/ClusterRunner,josephharrington/ClusterRunner,Medium/ClusterRunner,josephharrington/ClusterRunner,Medium/ClusterRunner,nickzuber/ClusterRunner,nickzuber/ClusterRunner,box/ClusterRunner