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/build.yml

@@ -13,12 +13,7 @@ jobs:
       with:
           # Nix Flakes doesn't work on shallow clones
           fetch-depth: 0
-    - uses: cachix/install-nix-action@v14
-      with:
-        install_url: https://github.com/numtide/nix-flakes-installer/releases/download/nix-2.4pre20210415_76980a1/install
-        extra_nix_config: |
-          experimental-features = nix-command flakes
-          system-features = nixos-test benchmark big-parallel kvm
+    - uses: cachix/install-nix-action@v15
     - name: List flake structure
       run: nix flake show
     - name: Run unit tests (flake)

flake.lock

@@ -2,11 +2,11 @@
   "nodes": {
     "nixlib": {
       "locked": {
-        "lastModified": 1635640270,
-        "narHash": "sha256-7VRyUAgl/Aqh70fkx904zciroUnz1cIUu0MqMaVu1pU=",
+        "lastModified": 1636849918,
+        "narHash": "sha256-nzUK6dPcTmNVrgTAC1EOybSMsrcx+QrVPyqRdyKLkjA=",
         "owner": "nix-community",
         "repo": "nixpkgs.lib",
-        "rev": "21595a2653dd84bbde02dadb0847a1bbb98a2663",
+        "rev": "28a5b0557f14124608db68d3ee1f77e9329e9dd5",
         "type": "github"
       },
       "original": {
@@ -17,11 +17,11 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1635934775,
-        "narHash": "sha256-DUkBfZjgeefgqyvFxnkZiOOWXgHP5Y1oKp/Zm+LT05Y=",
+        "lastModified": 1637186689,
+        "narHash": "sha256-NU7BhgnwA/3ibmCeSzFK6xGi+Bari9mPfn+4cBmyEjw=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "4789953e5c1ef6d10e3ff437e5b7ab8eed526942",
+        "rev": "7fad01d9d5a3f82081c00fb57918d64145dc904c",
         "type": "github"
       },
       "original": {