Default permissions for credentials are too wide

[slhck]

The default permissions allow read-access for everyone.

~/.gmcli using ☁️  default/
➜ ls -la
total 24
drwxr-xr-x@   5 werner  staff   160 Jan 18 12:50 .
drwxr-x---+ 226 werner  staff  7232 Jan 18 12:51 ..
-rw-r--r--@   1 werner  staff   357 Jan 18 12:49 accounts.json
-rw-r--r--@   1 werner  staff   149 Jan 18 12:49 credentials.json
-rw-r--r--@   1 werner  staff    41 Jan 18 12:50 default.json

These should be 600 by default to prevent other users from reading them.

You might want to cherry-pick slhck@9f4f1f4 for a possible fix.