Nice HLS called from wrapper.

[mgajda]

This is decreasing priority within hls-wrapper before it invokes HLS proper: #3528

We can also decrease priority in hie-bios for subordinate processes,
but going there would require much more effort, so is unlikely to land quickly.

See issues which may benefit ecosystem in the long term:

• Decrease priority of compilation jobs to increase UX responsiveness. hie-bios#399
• Decreasing process priority... process#281