community/chezmoi: upgrade to v1.7.18
This bumps chezmoi to version v1.7.18, which requires slightly different variables to embed the version number. See https://github.com/twpayne/chezmoi/commit/8284da771e8db09a6b9a4651154bb5f7820a62f0 and https://github.com/twpayne/chezmoi/pull/644.