docker: make sure nix config check works (backport #13351)

[mergify]

Motivation

This current PR addresses a small issue I encountered during the making of #13354.

When running nix config check inside the official Nix Docker image, the following failure was raised:

bash-5.2# nix config check
[PASS] PATH contains only one nix version.
[FAIL] Found profiles outside of /nix/var/nix/profiles.
The generation this profile points to might not have a gcroot and could be
garbage collected, resulting in broken symlinks.

  "/root/.nix-profile/bin"

[PASS] Client protocol matches store protocol.
[INFO] You are trusted by store uri: local

This PR proposes a fix for that issue. I would appreciate any feedback on the approach, and I am happy to adjust the implementation if needed.

Context

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This is an automatic backport of pull request #13351 done by [Mergify](https://mergify.com).