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]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@           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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