Write 0600 permissions when editing root config

[macmv]

Ticket(s): FE-4797

Problem

~/.fauna-shell contains secrets, so it should not be readable by everyone by default.

Solution

Write it with mode 0600.

Result

It is not readable by others by default.

Testing

I did fauna cloud-login, and the new file is only readable and writable by me:

$ ls -la ~/.fauna-shell*
-rw------- 1 macmv macmv  232 Dec  7 13:27 .fauna-shell
-rw-r--r-- 1 macmv macmv 1.5K Oct  9 10:49 .fauna-shell.bak
-rw-r--r-- 1 macmv macmv 1.5K Oct 12 15:40 .fauna-shell.old

[yesha-fauna]

Need to add warning if the fauna-shell or ProjectConfig Path does not have 600 permission.

[macmv]

looks good to me @yesha-fauna