dulwich: use GIT_SSH_COMMAND when set

[pmrowla]

Closes #160

When GIT_SSH_COMMAND or GIT_SSH are set, use the specified command (via dulwich's subprocess vendor) instead of our asyncssh vendor.

[codecov-commenter]

Codecov Report

Base: 80.92% // Head: 81.66% // Increases project coverage by +0.74% 🎉

Coverage data is based on head (5440102) compared to base (75e0b28).
Patch coverage: 66.66% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #161      +/-   ##
==========================================
+ Coverage   80.92%   81.66%   +0.74%     
==========================================
  Files          25       25              
  Lines        3397     3404       +7     
  Branches      586      587       +1     
==========================================
+ Hits         2749     2780      +31     
+ Misses        558      542      -16     
+ Partials       90       82       -8     

Impacted Files	Coverage Δ
src/scmrepo/git/backend/dulwich/__init__.py	79.13% <66.66%> (+0.10%)	⬆️
src/scmrepo/git/backend/gitpython.py	64.42% <0.00%> (+1.07%)	⬆️
src/scmrepo/git/backend/pygit2.py	76.48% <0.00%> (+1.98%)	⬆️
tests/conftest.py	95.00% <0.00%> (+7.00%)	⬆️
src/scmrepo/utils.py	87.09% <0.00%> (+19.35%)	⬆️

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