Enable cross-PR testing

Author: ahoppen

.github/workflows/pull_request.yml

@@ -8,9 +8,16 @@ jobs:
   tests:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
-  soundness:
-    name: Soundness
-    uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
     with:
-      license_header_check_enabled: false
-      license_header_check_project_name: "Swift.org"
+      enable_windows_checks: false
+      linux_pre_build_command: |
+        git clone https://github.com/swiftlang/swift-syntax.git ../swift-syntax
+        git -C ../swift-syntax fetch origin pull/2859/merge:pr_merge
+        git -C ../swift-syntax reset --hard pr_merge
+        swift package config set-mirror --original https://github.com/swiftlang/swift-syntax.git --mirror "$(realpath ../swift-syntax)"
+  # soundness:
+  #   name: Soundness
+  #   uses: swiftlang/github-workflows/.github/workflows/soundness.yml@main
+  #   with:
+  #     license_header_check_enabled: false
+  #     license_header_check_project_name: "Swift.org"