make busybox --install work even if /proc/self/exe doesn't exist

# size busybox_old busybox_unstripped
   text    data     bss     dec     hex filename
 680095    2704   15648  698447   aa84f busybox_old
 680099    2704   15648  698451   aa853 busybox_unstripped

shell/msh.c

@@ -3062,7 +3062,7 @@ static const char *rexecve(char *c, char **v, char **envp)
 			/* We have to exec here since we vforked.  Running
 			 * run_applet_and_exit() won't work and bad things
 			 * will happen. */
-			execve(CONFIG_BUSYBOX_EXEC_PATH, v, envp);
+			execve(bb_busybox_exec_path, v, envp);
 		}
 	}