Ensure env-vars are handled as strings

[ThomasEcuer]

This avoids suprocess.run to fail ambiguously due to non-string values inherited from configuration
(like postgresql' port number).

Cf. #83.

Successful PR Checklist:

• Tests
• Documentation (optionally: run spell checking)
• Had a good time contributing? (feel free to give some feedback)

[codecov]

Codecov Report

Merging #84 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #84   +/-   ##
=======================================
  Coverage   78.66%   78.66%           
=======================================
  Files          15       15           
  Lines         642      642           
  Branches       81       81           
=======================================
  Hits          505      505           
  Misses        127      127           
  Partials       10       10           

Flag	Coverage Δ
#integration	57.78% <100.00%> (ø)
#unit	60.90% <0.00%> (ø)

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

Impacted Files	Coverage Δ
septentrion/runner.py	100.00% <100.00%> (ø)

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[ewjoachim]

Released in https://pypi.org/project/septentrion/0.6.2/