Set `TF_CLI_ARGS_fmt` to avoid errors when it is already in the environment

[OJFord]

I use TF_CLI_ARGS_fmt=<path>, which points terraform fmt at the correct path without me having to specify it.

Unfortunately, it means it doesn't work with vim-terraform, since the executed command becomes terraform fmt <path> <path> and I get an error about only one arg being accepted.

To fix this issue, vim-terraform could either:

• run with TF_CLI_ARGS_fmt= blanked
• set TF_CLI_ARGS_FMT=<path> itself, and run with vanilla terraform fmt