build: Add artifacting of PR build

.github/workflows/test.yml

@@ -18,6 +18,12 @@ jobs:
           distribution: 'adopt'
           cache: gradle
 
+      - name: Get PR number
+        id: pr
+        run: echo "::set-output name=pull_request_number::$(gh pr view --json number -q .number || echo "")"
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
       - name: Validate Gradle wrapper
         uses: gradle/wrapper-validation-action@v1
 
@@ -26,4 +32,14 @@ jobs:
         with:
           arguments: build
         env:
+          GITHUB_VERSION: pr${{ steps.pr.outputs.pull_request_number }}
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Staging artifact
+        run: mkdir staging && cp build/libs/*.jar staging
+
+      - name: Artifact output
+        uses: actions/upload-artifact@v2
+        with:
+          name: multiverse-core-pr${{ steps.pr.outputs.pull_request_number }}
+          path: staging