Bump cachix/install-nix-action from 15 to 20

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 15 to 20.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](cachix/install-nix-action@v15...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>

Author: dependabot[bot]

.github/workflows/cache.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: actions/checkout@v3.4.0
 
       - name: "Install Nix️"
-        uses: cachix/install-nix-action@v15
+        uses: cachix/install-nix-action@v20
 
       - name: "Install Cachix️"
         uses: cachix/cachix-action@v10

.github/workflows/ci.yml

@@ -14,7 +14,7 @@ jobs:
       - uses: actions/checkout@v3.4.0
 
       - name: "Install Nix️"
-        uses: cachix/install-nix-action@v16
+        uses: cachix/install-nix-action@v20
 
       - name: "Install Cachix️"
         uses: cachix/cachix-action@v10