fix(bkgpslog): Fix broken working dir name

- Fix broken working dir variable (was missing script start time)

- Remove UTC as default TZ (will use parent shell's env var TZ). Can
  still be customized using `--time-zone` option

- Add logic for creating temporary working directory for possible
future `--temp-dir` option. Working dir hard defaults to
`/dev/shm` (tmpfs) for now.