Use cross-pr-testing from github-workflows

Author: ahoppen

.github/workflows/pull_request.yml

@@ -16,7 +16,7 @@ jobs:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
-      linux_exclude_swift_versions: "[{\"swift_version\": \"5.8\"}]"
+      enable_cross_pr_testing: true
   soundness:
     name: Soundness
     uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main