Describe the bug

When GH_CONFIG_DIR was introduced (#2444), the file help_topic.go was not updated, so gh help environment is not listing this important environment variable.

gh version 1.10.3 (2021-05-21)

Steps to reproduce the behavior

1. Type this 'gh help environment'
2. View the output
3. Mention to GH_CONFIG_DIR is missing

Expected vs actual behavior

The variable GH_CONFIG_DIR should me mentioned.