Fix flaky test tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-xdg-credential]

The merge lane saw `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-xdg-credential]` fail and then pass on a rerun of the same tree. - Test: `tests/test_git_mirror_isolation.py::test_a_planted_key_steers_no_daemon_git[create_worktree-xdg-credential]` - Tree: `97f5c44e1a99d35c0c115ffec4f12ddbf7381add` - Lane task: 7584 - Flake record: 01a151b7-920f-7363-a6cf-59f84365829c Make it deterministic; do not delete it unless it tests nothing.

Flake was host fork exhaustion: the fixture TLS server's thread start failed and the handshake was cut off. The server now retries EAGAIN spawns for up to 30s. Before the fix, injected EAGAIN reproduced the lane error; after it, 42/42 pass. Quick gate green. PR #6115 queued.