CI: Wrap nix devshell invocation with just

This commit also adds the flake version to the justfile's version check.

.forgejo/workflows/verify.yaml

@@ -16,10 +16,13 @@ jobs:
     container:
       image: rust:slim
     steps:
-      - name: Install action dependencies
+      - name: Set up action dependencies
         run: |
           apt-get install --no-install-recommends --update -y \
             nodejs git nix
+          echo experimental-features = nix-command flakes >> /etc/nix/nix.conf
+          echo 'export PATH=$PATH:$HOME/.nix-profile/bin' >> /etc/profile
+          nix profile install nixpkgs#just # trixie is on nix 2.26
 
       - name: Checkout code
         uses: actions/checkout@v6
@@ -48,8 +51,5 @@ jobs:
 
       - name: Assess
         run: |
-          ENV_PATH=$PATH su ci -c "
-            nix --experimental-features 'nix-command flakes' \
-              develop path:$PWD \
-              -c just -f $PWD/.justfile verify
-          "
+          command -v just
+          just ci-nix "$PATH" verify