feat: add support for env defined binary

[quotidian-ennui]

Resolves #67

Changes

Resolves a TF_BINARY environment variable that tells us what command to execute. Still assumes it's on the path so no behavioural difference.
Defaults to terraform for backwards compatibility.

Notes

• Choice of TF_BINARY was pretty arbitrary.
• I'm not primarily a go dev and there didn't seem to be an standardised way of get_env_or_default equivalent.
• I tested locally by executing the output from make build manually (so no additional tests).