Add go_env make target
Add go_env make target

This target shows a list of environment variables to export in the shell
to run custom go commands and tests.

pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia,pythia-project/pythia-core,pythia-project/pythia-core