BF: Do not maintain an environment copy in GitWitlessRunner

[mih]

Instead, adjust the current environment, as needed, at the time of
each run() invocation -- like GitRunner() does it.

This avoids silently ignoring changes in the process environment
after a GitWitlessRunner instance was created.

Fixes gh-4702

I will try this out in the context of #4699, which is an ideal testbed, due to the long-running ConfigManager instance.

[codecov]

Codecov Report

Merging #4703 into maint will increase coverage by 4.37%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##            maint    #4703      +/-   ##
==========================================
+ Coverage   85.22%   89.60%   +4.37%     
==========================================
  Files         288      288              
  Lines       39106    40172    +1066     
==========================================
+ Hits        33329    35995    +2666     
+ Misses       5777     4177    -1600     

Impacted Files	Coverage Δ
datalad/cmd.py	90.07% <100.00%> (+0.31%)	⬆️
datalad/support/tests/test_vcr_.py	57.89% <0.00%> (-3.22%)	⬇️
datalad/distributed/tests/ria_utils.py	44.73% <0.00%> (-1.21%)	⬇️
datalad/support/vcr_.py	49.12% <0.00%> (-0.88%)	⬇️
datalad/support/digests.py	100.00% <0.00%> (ø)
datalad/tests/test_base.py	100.00% <0.00%> (ø)
datalad/tests/test__main__.py	100.00% <0.00%> (ø)
datalad/ui/tests/test_base.py	100.00% <0.00%> (ø)
datalad/tests/test_protocols.py	100.00% <0.00%> (ø)
datalad/tests/test_dochelpers.py	100.00% <0.00%> (ø)
... and 163 more

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[kyleam]

I didn't do any testing on my end, but the problem description and proposed fix make sense to me.

[mih]

Thx!