Fix Coverity build to not use -Werror

[mkindahl]

The coverity build should not build with warnings-as-errors, so
disabling it for the build.

[mkindahl]

Building using coverity to check that it works: https://github.com/timescale/timescaledb/actions/runs/430736407

[codecov]

Codecov Report

Merging #2760 (920ec24) into master (d13afc7) will increase coverage by 0.11%.
The diff coverage is 90.00%.

@@            Coverage Diff             @@
##           master    #2760      +/-   ##
==========================================
+ Coverage   90.12%   90.24%   +0.11%     
==========================================
  Files         212      212              
  Lines       34640    34614      -26     
==========================================
+ Hits        31218    31236      +18     
+ Misses       3422     3378      -44     

Impacted Files	Coverage Δ
tsl/src/dist_util.c	93.28% <ø> (ø)
tsl/src/remote/connection.c	91.57% <88.57%> (-0.56%)	⬇️
tsl/src/data_node.c	96.25% <100.00%> (+0.02%)	⬆️
tsl/src/data_node_dispatch.c	97.02% <0.00%> (-0.25%)	⬇️
src/import/planner.c	70.30% <0.00%> (+11.12%)	⬆️

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