Bump DeterminateSystems/nix-installer-action in the nix-actions group (#382)

Bumps the nix-actions group with 1 update: [DeterminateSystems/nix-installer-action](https://github.com/determinatesystems/nix-installer-action).


Updates `DeterminateSystems/nix-installer-action` from 19 to 20
- [Release notes](https://github.com/determinatesystems/nix-installer-action/releases)
- [Commits](DeterminateSystems/nix-installer-action@v19...v20)

---
updated-dependencies:
- dependency-name: DeterminateSystems/nix-installer-action
  dependency-version: '20'
  dependency-type: direct:production
  update-type: version-update:semver-major
  dependency-group: nix-actions
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

.github/workflows/ci-nix.yml

@@ -26,7 +26,7 @@ jobs:
     timeout-minutes: 30
     steps:
       - uses: actions/checkout@v5
-      - uses: DeterminateSystems/nix-installer-action@v19
+      - uses: DeterminateSystems/nix-installer-action@v20
       - run: nix flake check
       - run: nix develop --command echo 'This step should be done before any other "nix develop" steps because of measuring Nix build time'
       - run: nix develop --command bundle install