We need to use an absolute path after chdir in run modes
where scripts aren't loaded into in-memory subs.
The oneshot test was also failing under TEST_RUN_MODE=0 due to
no "lei-oneshot" command existing on the FS. So we force a
socket failure by making XDG_RUNTIME_DIR too large to fit into
the 108-byte .sun_path field of "struct sockaddr_un". This
even lets us simplify lei-oneshot significantly.