Pass MAGEFILE_GOCMD to compiled binary (#210)
Pass MAGEFILE_GOCMD to compiled binary (#210)

Pass the `MAGEFILE_GOCMD` environment variable into the compiled binary
when running with `-gocmd` set.

Testing Done:

- Validated that running `mage -debug -gocmd <go-runtime> <target>`
  properly sets the environment variable `MAGEFILE_GOCMD=<go-runtime>`
  into the temporary binary.
- Validated that compiled magefiles will not persist the initial
  `-gocmd` when executing - though this is still toggleable by providing
  the environment variable manually.

fixes #208
magefile/mage,magefile/mage,magefile/mage