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/pr.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         extra_nix_config: |

.github/workflows/update.yml

@@ -14,7 +14,7 @@ jobs:
     - uses: actions/checkout@v2
       with:
         fetch-depth: '0'
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         extra_nix_config: |
@@ -28,7 +28,7 @@ jobs:
     needs: update_nur
     steps:
     - uses: actions/checkout@v2
-    - uses: cachix/install-nix-action@v14
+    - uses: cachix/install-nix-action@v15
       with:
         nix_path: nixpkgs=channel:nixos-unstable
         extra_nix_config: |