temporarily add back 5.9 CI

Author: glbrntt

.github/workflows/pull_request.yml

@@ -14,7 +14,7 @@ jobs:
         name: Unit tests
         uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
         with:
-            linux_5_9_enabled: false
+            linux_5_9_enabled: true
             linux_5_10_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors"
             linux_6_0_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors"
             linux_6_1_arguments_override: "--explicit-target-dependency-import-check error -Xswiftc -warnings-as-errors"