ci: Use newer version of actions/upload-artifact (#433)

v2 is now deprecated and causes the test run to fail.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

.github/workflows/test.yml

@@ -83,7 +83,7 @@ jobs:
     - run: make microwatt.json
     - run: make microwatt.bit
     - run: make microwatt.svf
-    - uses: actions/upload-artifact@v2
+    - uses: actions/upload-artifact@v4
       with:
         name: ${{matrix.task}}-bitstream
         path: microwatt.svf