go/lint: add optional check for code coverage threshold

[adamdecaf]

Each project can opt-in to threshold checking inside their workflow.

    - name: Check
      env:
        COVER_THRESHOLD: 85.0
      run: make check

[codecov-commenter]

Codecov Report

Merging #215 (fe0c468) into master (10a62bd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #215   +/-   ##
=======================================
  Coverage   78.37%   78.37%           
=======================================
  Files           1        1           
  Lines          37       37           
=======================================
  Hits           29       29           
  Misses          4        4           
  Partials        4        4           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 10a62bd...fe0c468. Read the comment docs.

[adamdecaf]

Project has 86.2% statement coverage.
SUCCESS: project has sufficient statement coverage