PlaceNixConfiguration: fix --extra-conf handling of extra-trusted-substituters and extra-trusted-public-keys

[cole-h]

Description

Fixes #1133.

Checklist

• Formatted with cargo fmt
• Built with nix build
• Ran flake checks with nix flake check
• Added or updated relevant tests (leave unchecked if not applicable)
• Added or updated relevant documentation (leave unchecked if not applicable)
• Linked to related issues (leave unchecked if not applicable)

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[donn]

Wait, why didn't this make it into 0.25.0?

[cole-h]

We got in a bit of a tizzy trying to get a release out with the Nix update and this got forgotten about in the heat of the moment. My apologies, but rest assured, it'll make it into the next release.

[grahamc]

@donn We have a handful of updates we're wanting to get out in the next couple days. We've recently adopted a policy to generally release installer changes separately from nix-installer changes, to reduce the likelihood of an issue with one blocking the release of the other.