buildx permissions on config files should be more permissive

I'm not sure how I ended up in this situation (perhaps I bind-mounted my homedir and switched contexts inside the container), but we probably can ease permissions on the files created within the config directory, as the user's home-directory itself should already be protected through permissions.

Here's what happened;

docker buildx imagetools inspect hello-world
ERROR: open /Users/thajeztah/.docker/buildx/current: permission denied

And lookiung at those files, they all seem to be either 0700 or 0600;

ls -la ~/.docker/buildx
total 16
drwx------   7 thajeztah  staff  224 Nov 21 21:24 ./
drwx------  17 thajeztah  staff  544 Nov 24 12:54 ../
-rw-------   1 thajeztah  staff   16 Sep  6 23:11 .buildNodeID
-rw-------   1 thajeztah  staff    0 Sep  6 23:11 .lock
-rw-------   1 root       staff   48 Nov 21 21:24 current
drwx------   2 thajeztah  staff   64 Sep  6 23:11 defaults/
drwx------   2 thajeztah  staff   64 Sep 29 17:48 instances/

Afaics, the files don't contain sensitive data, so wouldn't need to be very restricted;

sudo cat ~/.docker/buildx/current
{"Key":"desktop-linux","Name":"","Global":false}

The issue identifies ~/.docker/buildx/current and the surrounding config directory, but no repository file or test. Start by searching the Go code for creation of the buildx config files and inspecting their permission modes. Done means the relevant non-sensitive files can be accessed in the reported bind-mounted or context-switched scenario without loosening home-directory protection.