tool throws an error "Binary files are not found. Run build command instead"

[asannikov]

the tool cannot build correctly anymore because darwin 386 is not supported. See https://github.com/golang/go/issues/37610

related issue is here: https://github.com/jkawamoto/fgo/issues/15

i found only one place where it's possible to fix: https://github.com/jkawamoto/fgo/blob/master/command/update.go#L109

@jkawamoto could you say plz, is it correct? I can do pull request if you wish and if it's only one change.