Use joinpath() instead of checkink outout of tempdir()

[joa-quim]

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[codecov-io]

Codecov Report

Merging #295 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #295      +/-   ##
==========================================
+ Coverage   93.84%   93.87%   +0.02%     
==========================================
  Files          80       80              
  Lines        3656     3656              
==========================================
+ Hits         3431     3432       +1     
+ Misses        225      224       -1

Impacted Files	Coverage Δ
src/common_options.jl	95.89% <100%> (ø)	⬆️
src/gmt_main.jl	81.02% <0%> (+0.13%)	⬆️

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