Lockdown and upgrade the nix action versions (#2038)

Author: palfrey

.github/actions/prepare-nix/action.yaml

@@ -48,9 +48,13 @@ runs:
         fi
 
     - name: Install Nix
-      uses: >- # https://github.com/DeterminateSystems/nix-installer-action/releases/tag/v17
-        DeterminateSystems/nix-installer-action@21a544727d0c62386e78b4befe52d19ad12692e3
+      uses: >- # https://github.com/DeterminateSystems/nix-installer-action/releases/tag/v20
+        DeterminateSystems/nix-installer-action@786fff0690178f1234e4e1fe9b536e94f5433196
+      with:
+        source-tag: v3.13.0
 
     - name: Add Nix magic cache
-      uses: >- # https://github.com/DeterminateSystems/magic-nix-cache-action/releases/tag/v11
-        DeterminateSystems/magic-nix-cache-action@def9f5a5c6a6b8751c0534e8813a5d0ad2635660
+      uses: >- # https://github.com/DeterminateSystems/magic-nix-cache-action/releases/tag/v13
+        DeterminateSystems/magic-nix-cache-action@565684385bcd71bad329742eefe8d12f2e765b39
+      with:
+        source-tag: v0.1.6