internal/rosa/cmake: use DESTDIR instead of --prefix

Turns out --prefix is deeply broken, and DESTDIR works even when using ninja.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -893,7 +893,6 @@ func main() {
 			"with-toolchain", command.BoolFlag(false),
 			"Include the stage2 LLVM toolchain",
 		)
-
 	}
 
 	c.Command(