Pinning nix to 2.13.3 in github workflows

Nix 2.14 released with incompatibilities with cachix/install-nix-action.
This PR pins nix to 2.13.3 to avoid CI fails
See: cachix/install-nix-action#161

(cherry picked from commit 84533ae)

.github/workflows/mill-ci.yml

@@ -29,8 +29,9 @@ jobs:
         with:
           submodules: 'true'
 
-      - uses: cachix/install-nix-action@v14.1
+      - uses: cachix/install-nix-action@v19
         with:
+          install_url: https://releases.nixos.org/nix/nix-2.13.3/install
           nix_path: nixpkgs=channel:nixos-unstable
 
       - name: Coursier Cache
@@ -51,8 +52,9 @@ jobs:
         with:
           submodules: 'true'
 
-      - uses: cachix/install-nix-action@v14.1
+      - uses: cachix/install-nix-action@v19
         with:
+          install_url: https://releases.nixos.org/nix/nix-2.13.3/install
           nix_path: nixpkgs=channel:nixos-unstable
 
       - name: Coursier Cache
@@ -74,8 +76,9 @@ jobs:
           submodules: 'true'
 
       - name: install nix
-        uses: cachix/install-nix-action@v18
+        uses: cachix/install-nix-action@v19
         with:
+          install_url: https://releases.nixos.org/nix/nix-2.13.3/install
           nix_path: nixpkgs=channel:nixos-unstable
 
       - name: Coursier Cache