fix: always set GIT_CONFIG_GLOBAL to writable temp file

Summary

When running jobs in Docker containers as non-root, git commands fail to write to /root/.gitconfig (permission denied).

Root cause

GIT_CONFIG_GLOBAL was only set when FF_GIT_URLS_WITHOUT_TOKENS was enabled. For most users the flag is off, so git writes to $HOME/.gitconfig which is owned by root in Docker containers running as non-root.

Fix

Always export GIT_CONFIG_GLOBAL pointing to a writable temp file in writeExports(), regardless of feature flag. Also make setupTokenlessGitConfig() always create the seed file.