Clobbering the CLI config file by default

[brandonc]

If you don't specify the TF_CLI_CONFIG_FILE environment variable, multiple setup actions on the same github runner can clobber the credentials file written by previous actions. This is because, by default, setup-terraform overwrites $HOME/.terraformrc

setup-terraform seems to want to own the CLI config file that it creates in order to configure host credentials (it uses fs.writeFile, which overwrites the file if it exists) so a potential solution is to configure terraform with a temp file that has some uniqueness and can be cleaned up reliably. I'd be willing to create a PR, but I thought I'd ask: is there any reason to use the default $HOME/.terraformrc file path to configure terraform CLI in the absence of TF_CLI_CONFIG_FILE?