unix/Makefile: Fix regression using install on non-GNU systems.
unix/Makefile: Fix regression using install on non-GNU systems.

This was fixed previously in 31fc81d3b8bb227109777eed02d0021e564fb523 but
regressed in 4af79e76943312bf71be395a71fa80ab39e7c2c2.

Fixes #5885.

pramasoul/micropython,pramasoul/micropython,tobbad/micropython,adafruit/circuitpython,adafruit/circuitpython,henriknelson/micropython,pramasoul/micropython,henriknelson/micropython,MrSurly/micropython,adafruit/circuitpython,tobbad/micropython,MrSurly/micropython,tobbad/micropython,kerneltask/micropython,pramasoul/micropython,henriknelson/micropython,selste/micropython,adafruit/circuitpython,bvernoux/micropython,pramasoul/micropython,kerneltask/micropython,adafruit/circuitpython,bvernoux/micropython,adafruit/circuitpython,kerneltask/micropython,selste/micropython,MrSurly/micropython,bvernoux/micropython,bvernoux/micropython,selste/micropython,bvernoux/micropython,MrSurly/micropython,kerneltask/micropython,kerneltask/micropython,tobbad/micropython,selste/micropython,selste/micropython,MrSurly/micropython,tobbad/micropython,henriknelson/micropython,henriknelson/micropython