test: fix git_sandbox for git<2.33

[Apkawa]

for old git like git-2.30 the tests broke my ~/.gitconfig file and created thousands of name=GitHub lines

Description

I checked the git documentation and worked out a workaround for my situation. I think it should work for a fresh git too

ENVIRONMENT
       GIT_CONFIG
           Take the configuration from the given file instead of .git/config. Using the "--global" option forces this to ~/.gitconfig. Using the "--system" option forces this to
           $(prefix)/etc/gitconfig.

       GIT_CONFIG_NOSYSTEM
           Whether to skip reading settings from the system-wide $(prefix)/etc/gitconfig file. See git(1) for details.

       See also the section called “FILES”.

Checklist

• Add test cases to all the changes you introduce
• Run ./scripts/format and ./scripts/test locally to ensure this change passes linter check and test
• Test the changes on the local machine manually
• Update the documentation for the changes

Expected behavior

no touch my ~/.gitconfig

Steps to Test This Pull Request

run tests

Additional context

➜  ~ cat /etc/issue
Debian GNU/Linux 11 \n \l
➜  ~ git --version
git version 2.30.2

#638

[codecov]

Codecov Report

Patch coverage: 94.11% and project coverage change: -0.24 ⚠️

Comparison is base (e34de55) 98.11% compared to head (89978e1) 97.87%.

Additional details and impacted files

@@            Coverage Diff             @@
##               v3     #713      +/-   ##
==========================================
- Coverage   98.11%   97.87%   -0.24%     
==========================================
  Files          41       42       +1     
  Lines        1855     1928      +73     
==========================================
+ Hits         1820     1887      +67     
- Misses         35       41       +6     

Flag	Coverage Δ
unittests	97.87% <94.11%> (-0.24%)	⬇️

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

Impacted Files	Coverage Δ
commitizen/version_types.py	90.74% <90.74%> (ø)
commitizen/bump.py	100.00% <100.00%> (ø)
commitizen/changelog.py	99.49% <100.00%> (-0.51%)	⬇️
commitizen/cli.py	94.11% <100.00%> (ø)
commitizen/commands/bump.py	98.14% <100.00%> (+0.02%)	⬆️
commitizen/commands/changelog.py	98.91% <100.00%> (+0.02%)	⬆️
commitizen/defaults.py	100.00% <100.00%> (ø)

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