fix(nix): fix flake lock refresh

[SuperSandro2000]

Changes

This fixes a not fully tested change from #31921 which broke the new nix flake lock refresh for inputs named nixpkgs.

Context

DEBUG: currentDigest not found in string to replace (repository=NuschtOS/ixx, branch=renovate/flake-inputs)
       "stringToReplace": "nixos-unstable",
       "currentDigest": "5df43628fdf08d642be8ba5b3625a6c70731c19c"
DEBUG: Starting search at index 55 (repository=NuschtOS/ixx, packageFile=flake.nix, branch=renovate/flake-inputs)
       "depName": "nixpkgs"
DEBUG: Found match at index 55 (repository=NuschtOS/ixx, packageFile=flake.nix, branch=renovate/flake-inputs)
       "depName": "nixpkgs"
DEBUG: Digest is not updated (repository=NuschtOS/ixx, packageFile=flake.nix, branch=renovate/flake-inputs)
       "depName": "nixpkgs",
       "manager": "nix",
       "expectedValue": "9d3ae807ebd2981d593cddd0080856873139aa40",
       "foundValue": "5df43628fdf08d642be8ba5b3625a6c70731c19c"
 WARN: Error updating branch: update failure (repository=NuschtOS/ixx, branch=renovate/flake-inputs)
DEBUG: syncBranchState() (repository=NuschtOS/ixx, branch=renovate/crates)

Documentation (please check one with an [x])

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I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository