Don't allow writes to runtime files in /etc

We mistakenly bind-mounted the runtime /usr/etc files read-write in
/etc, which means that application could modify some parts of the
runtimes (at least when using a per-user installed runtime). Fix
this by using a --ro-bind.

(cherry picked from commit 08e47e9544)

common/flatpak-run.c

@@ -2464,7 +2464,7 @@ flatpak_run_setup_base_argv (FlatpakBwrap   *bwrap,
             }
           else
             {
-              flatpak_bwrap_add_args (bwrap, "--bind", src, dest, NULL);
+              flatpak_bwrap_add_args (bwrap, "--ro-bind", src, dest, NULL);
             }
         }
     }