add checking GOROOT environment variable

[t-matsuo]

If there is no GOROOT environment variable, 'operator-sdk add api'
fails.
see #1854 (comment)

Description of the change:

Motivation for the change:

[joelanford]

Technically, GOROOT only has to be explicitly set if the user's GOROOT is different than the GOROOT used to build the binary.

However, given that different maintainers create the release artifacts and that they have different environments themselves, there's no guarantee that the build GOROOT would be consistent release to release. There's also no way to determine what the build GOROOT was at runtime. So it seems to me that setting GOROOT is something most users who are not building from source are probably having to do.

There may be a way to solve this in the SDK binary itself, so that users do not need to worry about this at all. Something like the following is what I've been thinking of adding in the code generation code where this problem arises:

out, err := exec.Command("go", "env", "GOROOT").CombinedOutput()
if err != nil {
    // handle err
}
if err := os.SetEnv("GOROOT", string(out)); err != nil {
    // handle err
}

[t-matsuo]

I think solving it in the SDK binary itself is better than checking GORROOT too, so I will close this PR.
Thanks.

[joelanford]

@t-matsuo Would you be interested in working on a PR to solve it in the SDK binary?

[t-matsuo]

@joelanford I'm interested in it, but I am beginner at operator-sdk. I want someone to create a patch.