Fix #530 Hanging behavior with ASDF installed Go on Darwin

[StevenACoffman]

So this resolves the issue with ko hanging on Darwin when Go is installed using ASDF, however, I'm not sure if this is the final fix that still works for everyone else.

@jonjohnsonjr
Signed-off-by: Steve Coffman steve@khanacademy.org

[StevenACoffman]

Are there places where we actually want dir to be . instead of ""?

There are still a few more references to this g.dir where we don't do the song and dance like:
https://github.com/google/ko/blob/1ec35c80cf6964d950868a626374743d75cae169/pkg/build/gobuild.go#L690

Also, I wonder about just lifting the song and dance to where the creation of the struct in the first place:
https://github.com/google/ko/blob/1ec35c80cf6964d950868a626374743d75cae169/pkg/build/gobuild.go#L131

Or here:
https://github.com/google/ko/blob/1ec35c80cf6964d950868a626374743d75cae169/pkg/build/gobuild.go#L151

[jonjohnsonjr]

There are still a few more references to this g.dir where we don't do the song and dance

I think it's fine to leave those alone for now. This seems like a weird interaction with packages.Load that we need to workaround, but just execing go directly works as expected. I'll want to refactor some of this stuff regardless, so we can just merge this to fix things until I can look at it more closely.