gnarly linked multiple times causes issues

[cpuguy83]

When gnarly has been linked as docker multiple times there seems to be some weird issues that happen trying to execute the real docker.

1. We should unset gnarly-specific env vars so for whatever reason this case might come up we aren't re-processing the same thing (or set a special var so the newly expected gnarly knows not to process things).
2. Assuming 1 alone doesn't fix the problem, gnarly could try to test if the binary is a gnarly binary (like set some env var and expect gnarly to exit with some value on stdout).