aux/trace: add GALLIUM_TRACE_TRIGGER mode

aux/trace: add GALLIUM_TRACE_TRIGGER mode

similar to amd/radv driver debug modes for sqtt, this specifies a filename
which is checked on every flush(PIPE_FLUSH_END_OF_FRAME); when it exists,
the next frame (and only that frame) is captured into the trace

to use, specify a file with the env var, run your app, and 'touch /path/to/file'
when you want to capture a trace

Reviewed-by: Adam Jackson <ajax@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10058>