Pass MAGEFILE_GOCMD to compiled binary

[ihgann]

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

[ihgann]

@natefinch While this works, I have a few hesitations about it and don't know about the design you have in mind either.

If I have the following target in my magefile:

func CheckGoCmd() {
    fmt.Println(fmt.Sprintf("Go runtime is: %s", mg.GoCmd()))
}

Then the following output occurs:

> mage checkGoCmd
Go runtime is: go
> mage -gocmd /usr/local/bin/go checkGoCmd
Go runtime is: /usr/local/bin/go

These seem good.

> mage -gocmd /usr/local/bin/go -compile ./magew
> ./magew checkGoCmd
Go runtime is: go

This is still overwritable with the environment variable:

> MAGEFILE_GOCMD=/usr/local/bin/go ./magew checkGoCmd
Go runtime is: /usr/local/bin/go

tldr; the gocmd variable is not persisted into the compiled binary. I think this is proper in my opinion, as a binary creator might not want to establish the requirement of the go runtime being in a specific path. However, wanted to note this before you considered accepting, as I don't know the overall vision you might have for such a situation like this.

[natefinch]

Yeah..... the thing of it is, the MAGEFILE_GOCMD environment variable is really only meant for internal use. It wasn't really built to be used by magefiles themselves. I think it was a mistake to put it in the mg package.... but it's there now and I can't take it out, so I might as well make it consistent I guess. :)

I think this is all fine. It definitely should not be hard coded into the compiled binary.

[ihgann]

Cool - that makes sense. I'll probably switch to a different method locally (ideally if #24 ever gets implemented I can switch to args), but at least this should fix existing expectations.

[natefinch]

just FYI, I've been sick for the past 8 days, but I fully intend on reviewing this as soon as my brain is back to normal.

[natefinch]

👍