Makefile: define GOPATH

[alanchchen]

Before submitting a pull request, please make sure the following is done:

Checklist

• [ ] Fork the repository and create your new branch from master.
• [ ] Please mention the name of community in Pull Request title.
• [ ] Git Commit Messages
  • [ ] Use the present tense (Also in Pull Request title): "Add feature" not "Added feature"
  • [ ] Use the imperative mood (Also in Pull Request title): "Move cursor to..." not "Moves cursor to..."
  • [ ] Use rebase to squash/fixup dummy/unnecessary commits into only one commit.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9ec2bc7) 76.88% compared to head (e223e19) 76.88%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #289 +/- ## ======================================= Coverage 76.88% 76.88% ======================================= Files 146 146 Lines 11357 11357 ======================================= Hits 8732 8732 Misses 1866 1866 Partials 759 759 ```

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