ci: colorize tox

[mr-cal]

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• [x] Have you successfully run tox?

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Colorize the output of tox in Github CI.

Tox does not colorize its output in Github because of the envvar TERM is set to dumb. However, github handles colorized output quite nicely.

Upstream of https://github.com/snapcore/snapcraft/pull/4435 / https://github.com/canonical/craft-providers/pull/442

[codecov[bot]]

Codecov Report

Merging #145 (1623649) into main (6b79ebe) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main      #145   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           10        10           
  Branches         2         2           
=========================================
  Hits            10        10