actions: test {upload,download}-artifacts@v4 support

If Forgejo version is >= 7.0 and Forgejo runner > 3.3.0 artifacts@v4
are tested to work with a forked modified version that claims to be
compatible.

actions/example-artifacts-v4/.forgejo/workflows/test.yml

@@ -0,0 +1,47 @@
+on: [push]
+jobs:
+  upload-many:
+    runs-on: docker
+    steps:
+    - run: mkdir -p artifacts
+
+    - run: touch artifacts/ONE artifacts/TWO
+
+    - uses: https://code.forgejo.org/forgejo/upload-artifact@v4
+      with:
+        name: many-artifacts
+        path: artifacts/
+
+  download-many:
+    needs: [upload-many]
+    runs-on: docker
+    steps:
+    - uses: https://code.forgejo.org/forgejo/download-artifact@v4
+
+    - run: |
+        test -f many-artifacts/ONE
+        test -f many-artifacts/TWO
+
+  upload-one:
+    runs-on: docker
+    steps:
+    - run: mkdir -p path/to/artifact
+
+    - run: echo hello > path/to/artifact/world.txt
+
+    - uses: https://code.forgejo.org/forgejo/upload-artifact@v4
+      with:
+        name: my-artifact
+        path: path/to/artifact/world.txt
+
+  download-one:
+    needs: [upload-one]
+    runs-on: docker
+    steps:
+    - run: "! test -f world.txt"
+
+    - uses: https://code.forgejo.org/forgejo/download-artifact@v4
+      with:
+        name: my-artifact
+
+    - run: "test -f world.txt"