main: setup GJS profiler when GJS_TRACE_FD is set

This is the same environment variable that will be used in GJS to auto-connect Sysprof to the GJS profiler when the gjs binary is used.

By handling this in main(), we allow GJS to extract stack-traces and GC marks to be correlated with the frame data from cogl when run using sysprof-cli from a secondary VT. (See screenshot below).

sysprof-cli -c "/opt/gnome/bin/gnome-shell --wayland --display-server" --gjs --gnome-shell my-capture.syscap

If you do not want additional C-based stack traces (therefore, only javascript stack traces) add --no-perf to disable the use of perf_event_open() syscall for stacktraces.

and then to view the capture:

sysprof my-capture.syscap