Add a copy of the GPL to CI artifacts

The GPL requires that you include a copy of the GPL with every copy of
programs that are released under the GPL [1]. Before this change, CI
artifacts didn’t include a copy of the GPL.

Fixes #276.

[1]: <https://www.gnu.org/licenses/gpl-faq.en.html#WhyMustIInclude>

.github/workflows/build.yml

@@ -81,6 +81,9 @@ jobs:
       - name: Run ${{ matrix.build_type }} Unittests
         run: ctest --preset ${{ matrix.os.preset }} -C ${{ matrix.build_type }}
 
+      - name: Ensure artifacts contain LICENSE
+        run: cp LICENSE ${{ github.workspace }}/builds/${{ matrix.os.preset }}/Descent3/${{ matrix.build_type }}/
+
       - name: Upload Artifacts
         uses: actions/upload-artifact@v4
         with: