Bump cachix/install-nix-action from 19 to 20 (#597)

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 19 to 20.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v19...v20)

---
updated-dependencies:
- dependency-name: cachix/install-nix-action
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/ci.yaml

@@ -16,7 +16,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
     - run: nix-env -f default.nix -iA build
     - run: postgrest-docs-build
 
@@ -25,7 +25,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
     - run: nix-env -f default.nix -iA spellcheck
     - run: postgrest-docs-spellcheck
 
@@ -35,7 +35,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v3
-    - uses: cachix/install-nix-action@v19
+    - uses: cachix/install-nix-action@v20
     - run: nix-env -f default.nix -iA linkcheck
     - run: postgrest-docs-linkcheck