Add nix flake submodule prefetch

action.yml

@@ -1,4 +1,4 @@
-name: 'Initialize CI Environment and Repository'
+name: 'Initialize CI Environment and Repository for Flake Builds'
 description: 'Initialize a CI environment with nix (and also checkout the repository recursively with LFS)'
 inputs:
   token:
@@ -6,7 +6,7 @@ inputs:
     required: false
     default: ${{ github.token }}
   lfs:
-    description: 'Whether to enable lfs'
+    description: Whether to enable lfs. Note that nix flakes don't support LFS currently.
     required: false
     default: true
 runs:
@@ -27,4 +27,8 @@ runs:
         echo -e "machine $GIT_SERVER\nlogin oauth2\npassword ${{ inputs.token }}" > ~/.netrc
         # Disable clone protection to pull LFS as well
         GIT_CLONE_PROTECTION_ACTIVE=false git clone --branch $GITHUB_REF_NAME --recurse-submodules $GITHUB_SERVER_URL/$GITHUB_REPOSITORY .
-        git submodule update --init
+        git submodule update --init --recursive
+
+    - name: Prefetch git submodules in nix store
+      shell: "nix shell nixpkgs#nixVersions.nix_2_26 nixpkgs#git-lfs --command {0}"
+      run: git submodule foreach --recursive 'nix flake prefetch'