makefiles/color: fix build on NixOS

On NixOS, there is no `/bin/echo`, instead
`which echo` will return

    /home/benpicco/.nix-profile/bin/echo

so hard-coding the path will break here.

Use `/usr/bin/env` to get the path of `echo`.
This should be availiable on all systems at the same location.

makefiles/color.inc.mk

@@ -5,7 +5,7 @@ COLOR_GREEN  :=
 COLOR_RED    :=
 COLOR_PURPLE :=
 COLOR_RESET  :=
-COLOR_ECHO   := /bin/echo
+COLOR_ECHO   := /usr/bin/env echo
 
 # Check if colored output is not disabled by user, i.e: CC_NOCOLOR unset
 # or 0
@@ -22,7 +22,7 @@ ifneq ($(CC_NOCOLOR),1)
       COLOR_ECHO   := echo -e
       SHELL=bash
     else
-      COLOR_ECHO   := /bin/echo -e
+      COLOR_ECHO   := /usr/bin/env echo -e
     endif
   endif
 endif