pytest_plugin(fix[set_gitconfig]): Add GIT_CONFIG_GLOBAL for child processes

why: GIT_CONFIG is excluded from child process environment by git
what:
- Add GIT_CONFIG_GLOBAL env var which IS inherited by child processes
- Fixes submodule operations in strict build environments

Author: tony

src/libvcs/pytest_plugin.py

@@ -180,6 +180,7 @@ def set_gitconfig(
 ) -> pathlib.Path:
     """Set git configuration."""
     monkeypatch.setenv("GIT_CONFIG", str(gitconfig))
+    monkeypatch.setenv("GIT_CONFIG_GLOBAL", str(gitconfig))  # For child processes
     return gitconfig