rebuild-testbench.sh: explain BUILD_TYPE=native in usage() #794

Job	Run time
LP64-WIP	4m 48s
build-windows (imx8 imx8x imx8m)	11m 54s
build-windows (mtl)	11m 2s
build-linux (mnfst, imx8 imx8x imx8m)	5m 46s
build-windows (tgl tgl-h)	14m 2s
manifest-check	48s
build-linux (mnfst, mtl)	6m 16s
build-windows (-d, mtl)	12m 23s
build-linux (mnfst, lnl)	5m 25s
build-linux (mnfst, tgl tgl-h)	6m 54s
build-linux (zmain, imx8 imx8x imx8m)	6m 28s
build-linux (zmain, mtl)	7m 40s
build-linux (zmain, lnl)	5m 35s
build-linux (zmain, tgl tgl-h)	8m 42s
build-linux (-d, mnfst, mtl)	5m 42s
compare-linux-win	33s
	1h 53m 58s

Provide feedback