ci: Build a release tarball (Fixes #312)

On a pull request: build the tarball and add a comment to the PR with a
link to the resource.
On push to master: build the tarball and keep the archive accessible in
the associated GitHub Action.

.github/workflows/symlinks.yml

@@ -16,3 +16,36 @@ jobs:
           sudo apt install -y libgtk-3-dev --no-install-recommends
           ./configure
           make test
+      - name: Build png theme
+        shell: bash
+        run: |
+          sudo apt install -y libgtk-3-dev optipng --no-install-recommends
+          ./configure
+          make
+          cd build
+          tar -cf elementary-xfce.tar.gz *
+      - name: Upload theme artifact
+        if: ${{ github.event_name == 'push' }}
+        uses: actions/upload-artifact@v3
+        with:
+          name: elementary-xfce
+          path: build/elementary-xfce.tar.gz
+      - name: Get artifact link
+        shell: bash
+        run: |
+          echo 'ARTIFACT_URL=$(curl \
+          -H "Accept: application/vnd.github+json" \
+          -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" \
+          https://api.github.com/repos/${{ GITHUB_ACTION_REPOSITORY }}/actions/runs/${{ GITHUB_RUN_ID }}/artifacts \
+          | grep \"url\"' >> $GITHUB_ENV
+      - name: Add comment with link
+        uses: actions/github-script@v5
+        with:
+          github-token: ${{secrets.GITHUB_TOKEN}}
+          script: |
+            github.rest.issues.createComment(
+              issue_number: context.issue.number,
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              body: "Here's a link to the artifact: ${{ env.ARTIFACT_URL }}"
+            })