Fix registry permission to restart service if using sudo -u registry

What does this MR do?

Fix registry permission to restart service

When we run the registry commnads with the registry user and the current directory is not accessible by runit, it fails with:

fatal: unable to open current directory: access denied

Changelog: fixed

This problem only happens when using sudo -u registry as a prefix to the registry migration command.

Proposal

Change the directory as soon as we start to execute the command to a Dir where the registry user has access to: /var/opt/gitlab/registry. This will only happen for that process and will not move the user from its original location.

Now when system() spawns the child process, it inherits /var/opt/gitlab/registry as the working directory - which the registry user does have permission to access! ✅

The key insight: when you use sudo -u to switch users, the new user process inherits the current directory, but may not have permission to access it. You needed to ensure the working directory was somewhere the registry user could actually read.

For a deeper explanation of the problem, bit by bit, read the issue description: Registry migration fails to restart service whe... (#9423).

Related issues

Closes Registry migration fails to restart service whe... (#9423)

Checklist

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