Fix prof vernier child hangs

- Avoid the `vernier` wrapper because it injects `RUBYOPT` into child
  Ruby processes and can make nested Homebrew commands profile themselves.
- Load a scoped fork and exec guard for `brew prof --vernier` so child and
  replacement processes do not inherit active Vernier collector state.
- Use spawn for `Utils.popen` while profiling to avoid its manual fork path.
- Keep HTML profiler output from opening a browser unless stdout is a TTY.
- Add regression coverage for Vernier child process profiling.