ci: build and cache devenv output as well

Author: vlaci

.github/workflows/build-nix.yml

@@ -42,3 +42,5 @@ jobs:
           extraPullNames: pyperscan
       - run: |
           nix flake check -L --impure --option system ${{ matrix.arch }} --extra-platforms ${{ matrix.arch }}
+      - run: |
+          nix build -L --option system ${{ matrix.arch }} --extra-platforms ${{ matrix.arch }} .#packages.${{ matrix.arch }}.devenv