chore(deps): update determinatesystems/nix-installer-action action to v20

Author: renovate[bot]

.github/workflows/ci.yml

@@ -41,7 +41,7 @@ jobs:
         uses: cachix/install-nix-action@v31
       - uses: DeterminateSystems/magic-nix-cache-action@main
 =======
-        uses: DeterminateSystems/nix-installer-action@v19
+        uses: DeterminateSystems/nix-installer-action@v20
       - name: Build shell
         run: nix develop -c true
 >>>>>>> parent of d6ae2ad (switch to buildbot):.github/workflows/nix-github-actions.yml