Config should load as unmodified

[williammartin]

Description

This PR relates to cli/cli#8496

Since the config was moved to go-gh this bug has been lurking around. The idea behind IsModified is that the config file shouldn't be written if only hosts has changed, and vice versa. Unfortunately, if there is a hosts file then on load of the files, the config entry would be SetModified as a result of having the hosts being added as an entry.

The consequence of this is that any call to Write would attempt to write the config file, even if only hosts had changed.

This PR sets the top level Config as unmodified to avoid that.

Testing notes

This command should not write to the config file:

gh config set --host github.com boo bar

You can set the config file as immutable on Mac like so:

sudo chflags uchg ~/.config/gh/config.yml

and revert it with:

sudo chflags nouchg ~/.config/gh/config.yml

[williammartin]

Bumped this because it seemed to resolve some linting errors. cli/cli is already on 1.54+

[andyfeller]