build(deps): bump DeterminateSystems/nix-installer-action

Bumps [DeterminateSystems/nix-installer-action](https://github.com/determinatesystems/nix-installer-action) from 20 to 21.
- [Release notes](https://github.com/determinatesystems/nix-installer-action/releases)
- [Commits](DeterminateSystems/nix-installer-action@v20...v21)

---
updated-dependencies:
- dependency-name: DeterminateSystems/nix-installer-action
  dependency-version: '21'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

.github/actions/install-nix/action.yml

@@ -7,7 +7,7 @@ inputs:
 runs:
   using: composite
   steps:
-  - uses: DeterminateSystems/nix-installer-action@v20
+  - uses: DeterminateSystems/nix-installer-action@v21
     with:
       extra-conf: |
         accept-flake-config = true