nixos/wpa_supplicant: fix unit-start script

Ever since setting up bonding the `wpa_supplicant-unit-start` script has
been failing. This is because the file `bonding_masters` in
`/sys/class/net/` is *not* a directory containing `uevent`.

Adding a test to verify the `uevent` path to be sourced exists resolves
the problem.