Support for environment files in set_env

[gaborbernat]

Resolves #1938.

[codecov]

Codecov Report

Merging #2223 (8cf1ea7) into rewrite (cf1a2be) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           rewrite    #2223   +/-   ##
========================================
  Coverage    99.86%   99.86%           
========================================
  Files          162      162           
  Lines         9397     9434   +37     
  Branches       961      967    +6     
========================================
+ Hits          9384     9421   +37     
  Misses           3        3           
  Partials        10       10           

Flag	Coverage Δ
tests	99.86% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
src/tox/config/loader/ini/__init__.py	100.00% <100.00%> (ø)
src/tox/config/set_env.py	100.00% <100.00%> (ø)
src/tox/config/sets.py	100.00% <100.00%> (ø)
src/tox/pytest.py	100.00% <100.00%> (ø)
src/tox/tox_env/errors.py	100.00% <100.00%> (ø)
tests/config/test_set_env.py	100.00% <100.00%> (ø)

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[jugmac00]

LGTM with some annotations 🚀