Change PET branch used in CI tests (#1087)

#1065 introduced
changes to runtimes that PET needs to handle for tests to not fail
spuriously; this PR changes its branch to point to the `test.yml`
repository to avoid spurious failures.

The branch is based on [this
PR](open-web3-stack/polkadot-ecosystem-tests#524)
by @sigurpol that updates PET to handle the changes to
`pallet-staking/palletproxy`.

- [x] Does not require a CHANGELOG entry

Co-authored-by: fellowship-merge-bot[bot] <151052383+fellowship-merge-bot[bot]@users.noreply.github.com>

Author: rockbmb

.github/workflows/test.yml

@@ -538,6 +538,7 @@ jobs:
         uses: actions/checkout@v4
         with:
           repository: open-web3-stack/polkadot-ecosystem-tests
+          ref: runtimes-master
           path: ecosystem-tests
 
       - name: Install test dependencies