core: fix file permissions for "/var/lib/NetworkManager/secret_key"

Ooherwise, the file has wrong permissions:

  # ls -la /var/lib/NetworkManager/secret_key
  ----r-xr-x. 1 root root 50 May 14 13:52 /var/lib/NetworkManager/secret_key

Luckily, /var/lib/NetworkManager should be already

  # ls -lad /var/lib/NetworkManager
  drwx------. 2 root root 8192 May 14 13:57 /var/lib/NetworkManager

which mitigates this a bit.

Fixes: dbcb1d6d ('core: let nm_utils_secret_key_read() handle failures internally')

#175
(cherry picked from commit dc3a2f9b)
10 jobs for nm-1-18 in 41 minutes and 3 seconds (queued for 10 minutes and 5 seconds)
Status Job ID Name Coverage
  Test
passed #297680
allowed to fail
checkpatch

00:00:56

passed #297687
t_debian:9

00:16:50

passed #297689
t_debian:sid

00:15:46

manual #297688
allowed to fail manual
t_debian:testing
passed #297681
t_fedora:28

00:25:08

passed #297682
t_fedora:29

00:40:56

passed #297683
t_fedora:30

00:25:24

manual #297684
allowed to fail manual
t_fedora:rawhide
passed #297685
t_ubuntu:16.04

00:17:10

passed #297686
t_ubuntu:18.04

00:17:12