only change gopath when newGopath not exists in legacy mode

[CarlJi]

fix #247

[qiniu-bot]

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[codecov]

Codecov Report

Merging #248 (f45f938) into master (4b501e1) will decrease coverage by 0.14%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #248      +/-   ##
==========================================
- Coverage   71.10%   70.95%   -0.15%     
==========================================
  Files          34       34              
  Lines        2011     2011              
==========================================
- Hits         1430     1427       -3     
- Misses        473      474       +1     
- Partials      108      110       +2     

Flag	Coverage Δ
e2e-1.11.x	?
e2e-1.12.x	?
e2e-1.13.x	?
e2e-1.14.x	?
e2e-1.15.x	?
e2e-1.16.x	?
unittest-1.13.x	70.86% <50.00%> (-0.25%)	⬇️
unittest-1.14.x	70.95% <50.00%> (-0.15%)	⬇️
unittest-1.15.x	70.86% <50.00%> (-0.25%)	⬇️
unittest-1.16.x	70.86% <50.00%> (-0.25%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
pkg/build/tmpfolder.go	66.03% <50.00%> (-4.72%)	⬇️
pkg/build/legacy.go	100.00% <0.00%> (+4.87%)	⬆️

[CarlJi]

/assign @lyyyuna