silence warning which only appears when rebuilding manifest

[hishamhm]

Fixes #1446.

[codecov-commenter]

Codecov Report

Attention: 23 lines in your changes are missing coverage. Please review.

Comparison is base (4eee542) 79.07% compared to head (1f9561f) 84.72%.
Report is 3 commits behind head on master.

❗ Current head 1f9561f differs from pull request most recent head b32d306. Consider uploading reports for the commit b32d306 to get more accurate results

Files	Patch %	Lines
src/luarocks/cmd.lua	51.35%	18 Missing ⚠️
src/luarocks/core/cfg.lua	63.63%	4 Missing ⚠️
src/luarocks/util.lua	92.30%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1591      +/-   ##
==========================================
+ Coverage   79.07%   84.72%   +5.64%     
==========================================
  Files          89       94       +5     
  Lines       11218    11938     +720     
==========================================
+ Hits         8871    10114    +1243     
+ Misses       2347     1824     -523     

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