fix(proxies): define environment variable for proxy use for git clone

[jmkerloch]

When a proxy is needed for git clone, dulwich and high level interface porcelain only use environment variable for proxy definition : HTTP_PROXY / HTTPS_PROXY

In this PR we are :

• defining a wrapper for environment variable definition and backup
• use wrapper for all function using porcelain for git command

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 61.53846% with 10 lines in your changes missing coverage. Please review.

Project coverage is 70.35%. Comparing base (bf67a0f) to head (cfda597).
Report is 9 commits behind head on main.

Files with missing lines	Patch %	Lines
qgis_deployment_toolbelt/utils/proxies.py	47.36%	6 Missing and 4 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #584      +/-   ##
==========================================
- Coverage   70.43%   70.35%   -0.08%     
==========================================
  Files          47       47              
  Lines        3162     3188      +26     
  Branches      554      558       +4     
==========================================
+ Hits         2227     2243      +16     
- Misses        741      747       +6     
- Partials      194      198       +4     

Flag	Coverage Δ
unittests	69.79% <61.53%> (-0.07%)	⬇️

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

Files with missing lines	Coverage Δ
...loyment_toolbelt/profiles/profiles_handler_base.py	70.16% <100.00%> (+1.20%)	⬆️
qgis_deployment_toolbelt/utils/proxies.py	61.64% <47.36%> (-5.03%)	⬇️

[Guts]

LGTM!

[Guts]

Missing and too generic docstring.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud