dockerConfigFile.GetCredentialsStore.Store faulty symlink resolution behavior

### Description

When DOCKER_CONFIG points to a directory that does contain a relative, ~~dangling~~ `config.json` symlink, Docker credentials store will (apparently) wrongly resolve the link to the current working directory.

This can be reproduced with the following:

```
mkdir -p /tmp/foo
ln -s doesnotexist /tmp/foo/config.json
cd ~
DOCKER_CONFIG=/tmp/foo nerdctl login
cat ~/doesnotexist
ln -s /tmp/foo
```

This does suggest there is a bug in moby/docker somewhere where readlink is used to resolve against pwd instead of the parent dir (/tmp/foo).

In turn, if, for some reason, the file cannot be created in the current working directory (for example if it is readonly), this will error in a very baffling way with a very confusing message:
```
rename /tmp/TestBrokenCredentialsStore705218110/008/docker-config2430087685/config.json2911426040 doesnotexist: invalid cross-device link
```

First spotted in https://github.com/containerd/nerdctl/pull/3293#pullrequestreview-2231972713 although at the time I was unable to diagnose it.

While this is very likely a docker bug, we need to fix these tests to deal with that.

### Steps to reproduce the issue

_No response_

### Describe the results you received and expected

na

### What version of nerdctl are you using?

main

### Are you using a variant of nerdctl? (e.g., Rancher Desktop)

None

### Host information

_No response_