Fix "log.Fatalf will exit, defer file.Close() will not run"

[alexandear]

This PR fixes writeDefaultConfig() function.

[codecov[bot]]

Codecov Report

Merging #488 (7af6dd5) into master (b8f3808) will increase coverage by 0.12%. The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   68.70%   68.83%   +0.12%     
==========================================
  Files           9        9              
  Lines        1061     1062       +1     
==========================================
+ Hits          729      731       +2     
+ Misses        252      251       -1     
  Partials       80       80              

Files	Coverage Δ
runner/config.go	75.98% <28.57%> (+0.65%)	:arrow_up:
runner/engine.go	63.80% <0.00%> (+0.02%)	:arrow_up:

[xiantang]

Thanks