CI: fix artifact-building

The generic-build step runs an install to the host system; for artifact-
generation, we need it all centralized in a stage/ directory. Do that
separately for the KDE neon builds that produce the artifact.

.github/workflows/nightly-neon.yml

@@ -30,7 +30,9 @@ jobs:
         uses: calamares/actions/generic-build@v2
       - name: "Calamares: archive"
         working-directory: ${{ env.BUILDDIR }}
-        run: tar czf calamares.tar.gz stage
+        run: |
+          make install DESTDIR=${{ env.BUILDDIR }}/stage
+          tar czf calamares.tar.gz stage
       - name: "Calamares: upload"
         uses: actions/upload-artifact@v2
         with: