Update GitHub pull request workflow

Stop trying to test with Swift 5.9 on Linux

Author: lorentey

.github/workflows/pull_request.yml

@@ -9,7 +9,7 @@ jobs:
     name: Test
     uses: swiftlang/github-workflows/.github/workflows/swift_package_test.yml@main
     with:
-      linux_exclude_swift_versions: '[{"swift_version": "5.8"}]'
+      linux_exclude_swift_versions: '[{"swift_version": "5.8"}, {"swift_version": "5.9"}]'
       windows_exclude_swift_versions: '[{"swift_version": "5.9"}]'
 
   soundness: