use preferred encoding to decode environment variables

[dirk-thomas]

Fixes #92.

[codecov-io]

Codecov Report

Merging #95 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #95      +/-   ##
==========================================
+ Coverage   78.03%   78.08%   +0.05%     
==========================================
  Files          50       50              
  Lines        2777     2784       +7     
  Branches      463      463              
==========================================
+ Hits         2167     2174       +7     
  Misses        588      588              
  Partials       22       22

Impacted Files	Coverage Δ
colcon_core/shell/__init__.py	99.41% <100%> (+0.02%)	⬆️

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[mikaelarguedas]

looks reasonable to me.
Didnt test it locally but I assume it proved to fix the original locale issue