Bump cachix/install-nix-action from 18 to 20

Bumps [cachix/install-nix-action](https://github.com/cachix/install-nix-action) from 18 to 20.
- [Release notes](https://github.com/cachix/install-nix-action/releases)
- [Commits](https://github.com/cachix/install-nix-action/compare/v18...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>

.github/workflows/Cabal-Linux.yml

@@ -43,7 +43,7 @@ jobs:
 
       #  2020-08-01: NOTE: Nix instantiate still needed for HNix tests
       - name: "Install Nix"
-        uses: cachix/install-nix-action@v18
+        uses: cachix/install-nix-action@v20
 
       - name: "Repository update"
         run: cabal v2-update

.github/workflows/Cabal-macOS.yml

@@ -38,7 +38,7 @@ jobs:
 
       # Still required for Remote and some builtins
       - name: "Install Nix"
-        uses: cachix/install-nix-action@v18
+        uses: cachix/install-nix-action@v20
 
       # NOTE: Freeze is for the caching
       - name: "Configuration freeze"

.github/workflows/On-Release-Cabal-Linux.yml

@@ -31,7 +31,7 @@ jobs:
         run: sudo apt install libsodium-dev
       #  2020-08-01: NOTE: Nix instantiate still needed for HNix tests
       - name: "Install Nix"
-        uses: cachix/install-nix-action@v18
+        uses: cachix/install-nix-action@v20
       - name: "Repository update"
         run: cabal v2-update
       # NOTE: Freeze is for the caching

.github/workflows/On-Release-Nixpkgs-integration.yml

@@ -20,7 +20,7 @@ jobs:
           /nix/store
         key: ${{ runner.os }}-Nixpkgs-integration-test
     - name: "Install Nix"
-      uses: cachix/install-nix-action@v18
+      uses: cachix/install-nix-action@v20
       with:
         nix_path: "nixpkgs=channel:nixos-unstable"
     - name: "Run Nixpkgs integration test"

.github/workflows/Optional-Nix-dev-env-macOS.yml

@@ -24,7 +24,7 @@ jobs:
       with:
         submodules: recursive
     - name: "Install Nix"
-      uses: cachix/install-nix-action@v18
+      uses: cachix/install-nix-action@v20
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: "Install Cachix"

.github/workflows/Optional-Nix-dev-env-main.yml

@@ -68,7 +68,7 @@ jobs:
       with:
         submodules: recursive
     - name: "Install Nix"
-      uses: cachix/install-nix-action@v18
+      uses: cachix/install-nix-action@v20
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: "Install Cachix"
@@ -92,7 +92,7 @@ jobs:
       with:
         submodules: recursive
     - name: "Install Nix"
-      uses: cachix/install-nix-action@v18
+      uses: cachix/install-nix-action@v20
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: "Install Cachix"
@@ -125,7 +125,7 @@ jobs:
       with:
         submodules: recursive
     - name: "Install Nix"
-      uses: cachix/install-nix-action@v18
+      uses: cachix/install-nix-action@v20
       with:
         nix_path: nixpkgs=channel:nixos-unstable
     - name: "Install Cachix"