Fix: GITHUB_PATH should prepend

[jsoref]

closes #689

[codecov]

Codecov Report

Merging #690 (ea6ac6d) into master (0f04942) will increase coverage by 1.16%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##           master     #690      +/-   ##
==========================================
+ Coverage   49.27%   50.43%   +1.16%     
==========================================
  Files          23       23              
  Lines        2401     2534     +133     
==========================================
+ Hits         1183     1278      +95     
- Misses       1090     1116      +26     
- Partials      128      140      +12     

Impacted Files	Coverage Δ
pkg/container/docker_run.go	1.82% <0.00%> (-0.11%)	⬇️
pkg/common/git.go	54.10% <31.03%> (-5.69%)	⬇️
pkg/model/workflow.go	30.71% <33.33%> (+5.00%)	⬆️
pkg/model/planner.go	34.56% <41.37%> (+1.48%)	⬆️
pkg/container/docker_pull.go	36.17% <64.70%> (+17.98%)	⬆️
pkg/runner/step_context.go	71.99% <71.53%> (+3.04%)	⬆️
pkg/runner/run_context.go	79.74% <94.82%> (+3.33%)	⬆️
pkg/runner/command.go	90.58% <100.00%> (+2.35%)	⬆️
pkg/runner/runner.go	76.92% <100.00%> (+0.45%)	⬆️
... and 2 more

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