Auto merge of #42069 - QuietMisdreavus:low_pri, r=alexchrichton
Add an option to run rustbuild on low priority on Windows and Unix
This is a resurrection of #40776, combining their Windows setup with an additional setup on Unix to set the program group's *nice*ness to +10 (low-but-not-lowest priority, mirroring the priority in the Windows setup) when the `low_priority` option is on.