export GO_GIT_DESCRIBE_SYMBOL from env_dir

Removing the special case treatment of this entry fixes
building in environments without env_dir support.
work around incorrect $GIT_DIR on Heroku

It appears that `$GIT_DIR="."` in the deploy environment, while we're
not running in `/app/tmp/repo.git`, so any call to `git` fails.

This workaround hard-codes the absolute path, and frankly it seems like
a bad idea but I could not come up with anything better.