switch to github.sha so workflow works on pushes

The usage of github.event.pull_request.head.sha
only works on pushes in the PR context, however we
still want to run the integration test on merges
to ccip-develop. This change switches to using
github.sha which is available in all contexts.

.github/workflows/ccip-integration-test.yml

@@ -35,7 +35,7 @@ jobs:
       - name: Update chainlink-ccip dependency in ccip
         run: |
           cd ccip
-          go get github.com/smartcontractkit/chainlink-ccip@${{ github.event.pull_request.head.sha }}
+          go get github.com/smartcontractkit/chainlink-ccip@${{ github.sha }}
           make gomodtidy
       - name: Setup Postgres
         uses: ./.github/actions/setup-postgres