Bump cachix/install-nix-action from 14 to 15

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 14 to 15.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v14...v15)

---
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>

.github/workflows/test-flakes.yml

@@ -17,7 +17,7 @@ jobs:
       with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         install_url: https://github.com/numtide/nix-unstable-installer/releases/download/nix-2.4pre20210823_af94b54/install
         extra_nix_config: |

.github/workflows/test.yml

@@ -14,6 +14,6 @@ jobs:
     runs-on: ${{ matrix.os }}
     steps:
     - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
     - name: build
       run: NIX_PATH=nixpkgs=https://github.com/NixOS/nixpkgs/archive/nixpkgs-unstable.tar.gz nix-build --expr '(import <nixpkgs> { }).callPackage ./. { }'