Disable commit/tag GPG signing via a cached wrapper GIT_CONFIG_GLOBAL when
the developer's global gitconfig enables signing, clear inherited GIT_CONFIG_GLOBAL
in GitBinaryTestFixture, strip inherited TERM_PROGRAM* in Vitest setup, and
run ssh-posix-command-wrapper shell checks against an empty HOME/ZDOTDIR/XDG_CONFIG_HOME.
Fixes#27210
Co-authored-by: outof-place <contact@outofplace.space>