ci(interop): Use run-interop-ping-test master latest hash (#3462)

With libp2p/test-plans#121 merged, we should be able to use @master
directly. Still pointing to a concrete git hash.

This includes
libp2p/test-plans@6d1aed2, thus
allowing interop tests to run from fork pull requests.

.github/workflows/interop-test.yml

@@ -18,7 +18,7 @@ jobs:
       - uses: docker/setup-buildx-action@v2
       - name: Build image
         run: docker buildx build --load -t rust-libp2p-head . -f interop-tests/Dockerfile
-      - uses: libp2p/test-plans/.github/actions/run-interop-ping-test@482e8ac2e5d91c69d7f432d325337e2ddca39cf9
+      - uses: libp2p/test-plans/.github/actions/run-interop-ping-test@2b40c181de24689f2ff29f2a595e46601c32b828
         with:
           test-filter: rust-libp2p-head
           extra-versions: ${{ github.workspace }}/interop-tests/ping-version.json