probe defaults cause issues with svt-av1

[gibbly]

Using preset 12 on svt-av1 with some settings causes blocking artifacts, particularly with 10 bit color and low crf values. This leads to vmaf calculations being wildly incorrect and causes av1an to pick inappropriately low crf values for effected scenes when using target-quality

Easy fix: set the preset for probes to 8, which doesn't seem to have this issue

Better fix: make speed/preset for probes user configurable

These were encoded at preset 12, crf 4, and pix_fmt yuv420p10le

[luigi311]

you can use --probe-slow as a work around as it will use your encoding settings for the probes.