ci: nix dev true (#91)

Author: ryoppippi

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

@@ -20,3 +20,7 @@ runs:
         restore-prefixes-first-match: |
           nix-${{ runner.os }}-${{ inputs.python-version }}-
           nix-${{ runner.os }}-
+
+    - name: Verify Nix installation
+      shell: bash
+      run: nix develop --command true